Keyguard: only show after screen fully turned off

Fixes an issue where the keyguard was displayed before the screen
was turned off fully, causing the AOD screen to briefly show during
the screen off animation.

Change-Id: Idc4621995bd67c1db255fab5a8de60d3aeef5d18
Fixes: 63526727
Test: Turn screen off. Make sure the AOD clock is not visible