Pin bubbles to the bottom right on tablets for the prototype

- Remove some old code around bubbles in taskbar that is not currently
  relevant
- Update some of that code to work for the prototype, namely set
  a pinned location while the prototype is running and modify touch
  handling behavior so that the stack remains pinned

Test: manual - turn on the flag and add a bubble on tablet
             => bubble is above the taskbar on the bottom right
             => dragging and releasing the bubble returns it back
                to the same position
             => you can drag to dismiss when collapsed or expanded
Bug: 253318833
Bug: 173386799
Change-Id: I1a9530f98d908e19c5e65ffee9a87459a3e51c8c