base

Partially fix the keyguard scrim

Currently the scrim is effectively translucent because
PWM doesn't consider it to be hiding windows behind it.
This change fixes that but the transition into the scrim
still leaks a few frames when SystemUI crashes.

Bug: 16165504
Change-Id: Id50b7a5c96df22dcae7705b4a071e7bbf64c2c5f