animation: fix unintentional loop while polkit dialog is active

The polkit password dialog has a spinner that gets displayed while the users password is being verified.

Unfortunately, the spinner stop method unintentionally calls back into itself after the stop fade out animation is complete. The stop method is called at startup, so the looping begins as soon as the dialog is visible and continues until the dialog is dismissed.

This commit fixes the loop by having the stop method cease calling itself, and instead having it call the stop method on the superclass.