This fixes a race condition seen whenever show() is called before the timer in hide() triggers. This can happen
for example if the user hits "Emergency Dial" followed by the back button before the 0.5s timeout completes. The result
is a blank screen with no keyguard view and no means to recover on devices w/o a keyboard.
The bug caused us to sometimes remove the newly created KeyguardView instead of the old one, leaving the view empty.
The fix is to always remove the last view.
6c0a563e67