gs-application.c: Show update preferences if flatpak enabled (LP: #1791553)
requested to merge ahayzen/gnome-software:3-30_enable_update_preferences_for_flatpak into ubuntu-3-30
@robert.ancell This is a version of !5 (merged) for the 3-30 branch.
Note I did the check after gs_application_initialize_ui as that is where the plugins are init'd (as this method then calls gs_application_initialize_plugins).
I have tested this on a cosmic machine with/without the flatpak plugin loaded and it acts as I would expect.