gdm/util: Only initialize fingerprint and smart card managers if needed

gdm/util: Only initialize fingerprint and smart card managers if needed

There's no point to initialize the fprint proxy and the smartcard
manager if they are disabled in authentication settings, so just avoid
initializing them, but at the same time this implies tracking of user
changes and so:
 - If a new service has been enabled, we initialize it
 - If a service has been disabled we destroy it and reset the
   authentication if such service was currently active

In both cases we do update the default service.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2267>