Decouple taskbar background from overveiw scrim if taskbar is in overview

- Removed all forceHideBackground calls if taskbar would be in overview
- Make overview scrim transparent regardless of stash state if taskbar would be in overview

Fix: 234139692
Test: Go to overview from app with ENABLE_TASKBAR_IN_OVERVIEW on/off and taskbar stashed/unstashed
Change-Id: I29e811cb93894e6c9e7a75c0ea493719f15eeccb