XdpAppInfo: Centralize handling of xdp_app_info_new_host ()
XdpAppInfo: Centralize handling of xdp_app_info_new_host ()

In the flatpak case, handle the not-a-flatpak case by returning
NULL (with no error set) so that we can continue trying any
other alternatives, with a single last fallback for the uncontained
host case.

matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal