util: abstract out default auth service in code

Right now, the primary way a user logs in is with
a password. They can also swipe their finger, if their
fingerprint is enrolled, but it's expected the fingerprint
auth service won't ask questions the user has to respond to
by typing. As such, we ignore questions that comes from
anything but the main auth service: gdm-password.

In the future, if a user inserts a smartcard, we'll want
to treat the gdm-smartcard service as the main auth service,
and let any questions from it get to the user.

This commit tries to prepare for that eventuality by storing
the name of the default auth service away in a _defaultService variable
before verification has begun, and then later checking incoming
queries against that service instead of checking against
string 'gdm-password' directly.

Of course, right now, _defaultService is always gdm-password.

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