Adds BubbleControllers to TaskbarControllers

* BubbleControllers is optionally available (gated behind a flag).
* The bubble bar controllers follow init & onDestroy after the taskbar
  controllers.
* Adds the relevant views used by the bubble controllers to the
  transient taskbar layout.

Bug: 253318833
Test: manual, with other CLs, see go/bubble-bar-tests
Flag: WM_BUBBLE_BAR
Change-Id: I685163cdcc912768b1a720952762f4cbc602b14b