Remove QS background

QS background was removed and elements around it were adjusted to match
the new Silk style.

This includes showing a scrim when pulling it down from the lock screen,
fading away the clock, and changing the color of some UI elements.

Fixes: 173562510
Test: atest ScrimControllerTest
Test: atest KeyguardClockPositionAlgorithmTest
Test: manual
Change-Id: Ie995121d85db9034860f4a70182d6328b4b6f0b3