Move Taskbar background drawing to TaskbarBackgroundRenderer

Currently used by both TaskbarDragLayer and TaskbarScrimView

Test: stash and unstash taskbar, background drawn as before; open a
Bubble and ensure taskbar is still scrimmed
Bug: 225200928

Change-Id: I3f7f4c1b1232b53c3eacb0e000d1c3aec25bb49d