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".
1 job for usb-app-installation in 16 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #302010
build-gnome-software

00:16:27