[flexi] Add LockscreenBehindScrim

Handles scrim over the lockscreen wallpaper
for better text contrast on the lockscreen.

Also adds an extra dim for the AOD
wallpaper in lowlight conditions.

Bug: 433477971
Test: atest DozeScreenBrightnessTest
Test: visually observe lockscreen scrim
Flag: com.android.systemui.scene_container
Change-Id: Icc2c164c4c8d1c668f83429b400f3ea9c34fa371