flatpak: add update_remote_sync call to appstream refresh
This applies changes to the flatpak remote which come from the summary file, such as updated keys, titles or the URL. Adding it into the appstream refresh function means it's behind the timestamp checks and skipping broken remotes, which avoids adding network roundtrips to the refresh. libflatpak caches the summary file in the common case that the remote details have not changed.
Fixes issue #293 (closed) .