flatpak: do not fetch updates every time we check the size of a package

It makes libflatpak (libsoup internally) go bonkers. It also makes the
whole startup much slower (understandably).
By using FlatpakRemoteRef instead of querying the FlatpakInstallation we
have the opportunity for some optimisations since we're querying the
FlatpakRemoteRef twice for some (most?) available packages.