loginDialog: Ensure old timed login timeout is removed before starting a new one

loginDialog: Ensure old timed login timeout is removed before starting a new one

Normally, we give the user a 5 second grace period of inactivity before
starting a timed login operation. Unfortunately, that grace period
timeout isn't properly removed if the timed login operation is restarted
during the grace period. That means the timeout handler can
inadvertently get called multiple times leading to the grace period
duration getting subtracted from the total animation time more than
once.

This commit ensures we only ever have one grace period timeout scheduled
at a time.