New scrim behind bubble stack

Top to bottom
1. [Scrim in front]
2. Status bar
3. [Scrim behind] (moved above bubble stack)
4. Bubble stack
5. [Bubble scrim] (new)

Fixes: 130237686
Test: atest SystemUITests
Test: manual: expand stack (see bubble scrim), then pull down shade (see
scrim behind status bar)
Change-Id: I2f39f41c524af2134c383c3e12dd193cbc487d93