Bubbles taskbar broadcast

* listen for broadcasts related to taskbar
* update info in BubblePositioner based on that
* show a scrim behind the bubbles when taskbar is on left / right so
  you don't see contents beneath

Test: manual
Bug: 173386799
Bug: 167413172
Change-Id: Id520718e779d3a45bcde447d7fcb0d2ba785c88e