Rework cast slider.

This CL improves Volume panel behaviour when casting. It makes currently active volume slider to be on top and simplifies the underlying API.

Flag: aconfig new_volume_panel TEAMFOOD
Test: manual on the phone
Fixes: 329641812
Fixes: 329561499
Change-Id: I7569e353d72ad8759e0b1367168b42e4e2a71d6f