Hide wallpaper when backdrop arrives

Some apps may set a backdrop image after the device enters doze,
causing the artwork to be visible in AOD. Let's draw black.

Test: manual
Test: packages/SystemUI/tests/src/com/android/systemui/statusbar/phone/ScrimControllerTest.java
Change-Id: Ib13e14f285594349c1df263f713883449884ff00
Fixes: 110959983