gdm/util: Use fully async call to setup the fingerprint device proxy

gdm/util: Use fully async call to setup the fingerprint device proxy

Since fingerprint service can now be started also if a conversation has
already began, we can also initialize the proxy asynchronously, without
the risk that the service won't be started early enough.

As per this, remove the usage of FprintDeviceProxy wrapper completely
since it's just not giving us anything here.

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