Introduce scrim state for bubbles

When bubbles are expanded we'll want to put a scrim behind them, this
is the scrim state we'll use.

Super simple test to check that alpha / visibility state of scrims
are as expected

Test: atest frameworks/base/packages/SystemUI/tests/src/com/android/systemui/statusbar/phone/ScrimControllerTest.java
Bug: 111236845
Change-Id: I1ee8ff9e7ce35aae692cf75a912106255e0e9b5a