flatpak: Add apps to the USB category
Flatpak supports repositories in removable drives in order for users to update apps from such devices. Temporary remotes can be listed by libflatpak but instead of showing a new GsApp object for each ref in those remotes, we should instead show the matching apps that come from statically configured remotes (because of how the installation/update of apps from USB remotes is designed). Thus, these changes set up the collection ID when creating the AsApp objects from refs in regular static repos so they can be fetched by refs coming from USB remotes that match have the same name, branch, and collection ID. The matched AsApp objects are given the USB category and keyword, so the resulting GsApp objects after loading the AppStream store) will also get that category and show up in search results when searching for "usb".