Wake the screen on alarm.

The alarm alert window style *must* be fullscreen in order
to successfully turn the screen on (even with the
appropriate window flags). To accomplish this but still look
like a dialog, AlarmClock has historically had a fake dialog
backdrop that it uses; we employ the same technique here
again.

Bug: 3505688
Change-Id: I66511f1b6f17c9ec61d430883caa234f18b704cf