Decouple taskbar background color from overview

- Taskbar background is 15L but Overview dark should be 35L

Test: Swipe up from taskbar (which is 15L), background matches until
going to overview, then animates to appropriate overview color based on
theme (light or dark).
Bug: 227344452
Change-Id: I52d892a569c8db565b4cfe271e532009f34ec669