gdmUtil: pave way for fingeprint to optionally be default auth service

Currently, fingerprint authentication is always a secondary thing.
If a user wants to swipe their finger when the computer is asking
for a password, so be it.

This commit paves the way for making fingerprint auth optionally
be the main way to authenticate.  Currently there's no way to enable
this, but in a future commit will honor

enable-password-authentication=false

in gsettings.

https://bugzilla.gnome.org/show_bug.cgi?id=683437