authPrompt: stop spinner after its hidden

authPrompt: stop spinner after its hidden

The code previously tried to stop spinner after it was hidden, but
due to an incorrect check was only stoppig it after it was shown.

Also, it was only stopping after hiding due to an animation, and
failing to stop it in the non-animated case.

This left the spinner hidden and running while VT switched away
from the login screen, only stopping when the auth prompt was
reset when switching back.

https://bugzilla.gnome.org/show_bug.cgi?id=753891