loginDialog: Correct source name for timed login idle timeout

loginDialog: Correct source name for timed login idle timeout

The _blockTimedLoginUntilIdle method sets a timeout to be called after
the user is idle for 5 seconds.  That timeout is erroneously given the
source name "[gnome-shell] this._timedLoginAnimationTime" which looks
like a copy-and-paste mistake.  The original intention was probably to
use a source name of "[gnome-shell] this._timedLoginIdleTimeOutId" which
more closely matches existing convention for source names.

This commit fixes that.