Allow bubbles to be displayed at the bottom of the screen

* Positions the bubbles to show at the bottom of the screen, above the
 'dismiss' target UI by default
* Adds flag to toggle bubbles to show at the top of the screen instead
* Adds flag to show bubble header as a footer instead

Test: manual - have bubbles, expand them, note that they are positioned at
      bottom of screen
Bug: 123540009
Change-Id: I938d3ab7d4ec6f6bd88f3a33f73a97d5f80a45f5