Unfortunately, my research has left me unable to give you a definitive answer here, but I can perhaps shed some light about the topic, and about the research methods I used to investigate this.

First of all: King Louis XVIII and Jules Jean Baptiste Anglès were real people, and Anglès did serve as prefect of police from September 1815 to December 1821. His name pops up regularly on documents from the period. Not knowing at this point whether that precise report as relayed by Victor Hugo was real, I can tell you that the Parisian police prefect did write confidential reports for the king and his government — as, indeed, did a whole host of French government officials during this time. All departmental prefects were appointed by the government in Paris, and part of their responsibilities was to keep the king and his ministers informed of potential unrest in their territories. (This included both actual civil unrest, and what we today would term normal political dissent.) And the style of the alleged report is roughly consonant with how people wrote at the time, though perhaps one could read it as being slightly on the florid side.

And whether or not Anglès actually wrote what Victor Hugo said he wrote, it's entirely plausible that Anglès believed it in the year 1817. That was a year of considerable unrest in France, following terrible harvests the year before (the "Year Without a Summer" brought on by a volcanic eruption). Many rural parts of France saw huge spikes in crime, and even organized armies of bandits raiding towns in search of grain. But Louis XVIII's government prioritized shipping food into Paris (even at the expense of aggravating shortages in rural areas), and so the people of Paris remained quiescent even in the midst of massive civil unrest elsewhere in the country. "The populace is restless in the provinces; it is not in Paris" — true on both accounts! Given the context that two decades prior the people of Paris had regularly brought down governments, Anglès might have felt justified in asserting that "there is nothing to be feared from these people" — though in the long run, past 1817, the people of Paris would prove far from quiescent.

So nothing about this alleged report is facially implausible, or would lead us to immediately reject it out of hand. Now we turn to the question of seeing if we can prove it was real.

The first step is to obtain the original quote. Both Hugo and Anglès wrote in French, so we need to track down a French version of Les Misérables. This is pretty easy — Project Gutenberg has an HTML version of the original novel, and we can easily go to Livre troisième, Chapitre V—Chez Bombarda, which gives us this quote:

C'était un temps de paix incontestable et de profonde sécurité royaliste; c'était l'époque où un rapport intime et spécial du préfet de police Anglès au roi sur les faubourgs de Paris se terminait par ces lignes: «Tout bien considéré, sire, il n'y a rien à craindre de ces gens-là. Ils sont insouciants et indolents comme des chats. Le bas peuple des provinces est remuant, celui de Paris ne l'est pas. Ce sont tous petits hommes. Sire, il en faudrait deux bout à bout pour faire un de vos grenadiers. Il n'y a point de crainte du côté de la populace de la capitale. Il est remarquable que la taille a encore décru dans cette population depuis cinquante ans; et le peuple des faubourgs de Paris est plus petit qu'avant la révolution. Il n'est point dangereux. En somme, c'est de la canaille bonne.»

Even if one doesn't read French, one can pick up enough proper nouns and cognates to match that with your English translation — "profonde sécurité royaliste" for "profound royalist security," "indolents" for "indolent," "grenadiers," etc.

Now one can search for excerpts of that, and see what comes up. In my searching, all hits are from Hugo's book (except for one mislabeled book that claims to be from Dumas' The Count of Monte Cristo, but is actually just Les Misérables). That doesn't prove anything in particular — a quote from an immensely popular and public domain work is going to swamp any actual collection of 1817 police reports in a Google search! So we need to turn to more specialized research.

Enter Gallica, the online document repository of the French National Library, the Bibliothèque Nationale de France. They have a tremendous amount of period documents online for free perusal, and it's pretty easy to research if you know a little French. (Fluency helps, but isn't necessary.) For example, here is a scanned volume of all the circulars and other instructions sent from the Ministry of the Interior to officials in the departments from 1807 to 1815.

Unfortunately, my searches on Gallica don't turn up anything of use, either. Again that proves nothing, but in perusing results for "Anglès" or for terms like "rapport au roi" I didn't find anything that happens to match this confidential report Hugo was ostensibly quoting.

Broadening my search somewhat, I found the book Victor Hugo et La Restauration: Étude Historique et Littéraire, published in 1869 in French by Edmond Biré. It is, at least in part, a fact-check of Les Misérables's statements about history. In general, M. Biré finds the book, which (he says elsewhere) "has the pretension of being a page from history," to be in fact full of errors both numerous and serious, which he documents at length. For example, in Volume II, Book 3, Chapter VI, Hugo writes, "At that period, King Louis XVIII went nearly every day to Choisy-le-Roi: it was one of his favorite excursions." But Biré, fact-checking, finds that the government-run newspaper "never failed to indicate how the king spent each of his days," and finds that in the month in question, Louis only left his palace on six days, and only one of them to Choisy. Unfortunately Biré does not appear to have researched the alleged Anglès report — there are scant references to the Comte d'Anglès in his writings.

But we must interrogate our sources! Jean Baptiste Edmond Biré was an author and critic in the final decades of the 19th Century. He was also, per French Wikipedia, a "legitimist" — that is, a member of the French right-wing Catholic royalist movement — and the father and grandfather of right-wing politicians. Given that Victor Hugo, by the time he published Les Misérables in the 1860s, was a champion of the French Left (though as a young man in the 1820s he was royalist, for a time), Biré's fact-check might seem to be politically motivated. That doesn't mean it's wrong, only that we should consider Biré's critique carefully.

Moreover, critics with fewer axes to grind have also highlighted issues with Victor Hugo's historicity in what is, we must remember, a novel, not a history book. Norman Denny, the translator of my edition of Les Misérables, writes in his introductory preface that while Hugo "pored endlessly over maps and documents" in researching his novel, he was often inaccurate — and singles out Volume I, Book 3, the book that contains "Chez Bombarda":

Hugo has sought to convey the social climate of that particular year by compiling a lengthy catalogue of personalities and events, most of them of no great importance — people and happenings, in short, that got into the news at the time... As for the present day, [translator] Professor Guyard has found it necessary to append sixty-two footnotes for the enlightenment of contemporary French readers — incidentally pointing out, not infrequently, that Hugo got his facts wrong.

I wasn't able to track down the Guyard edition (Denny excised most of the footnotes mentioned above from his English translation), but I did look up several different editions of the novel that contain footnotes and found no notes about the ostensible Anglès report.

So what are we to make of all this? This police report from Hugo's novel is plausible, but there is zero evidence to back it up, and we know that Hugo got other historical facts wrong. Of course, misstating how often the king went on carriage rides is not quite the same thing as fabricating an entire police report. But authors of fiction can and do take literary license when history doesn't provide the material they need for their story. The authenticity of this quote remains unclear, but with the absence of evidence and other historical flaws, there are plenty of reasons to doubt that the Comte d'Anglès wrote verbatim about the "amiable rabble."