Shift bubbles for IME on large screens

* Incorporate IME state when calculating the x/y position
  of the bubbles -- if the IME is up, shift the bubbles
  so they don't overlap it (if possible).
* When the IME visibility changes:
    - positioner saves the state
    - stackview animates the bubbles to their new position

Test: manual - open the IME with bubbles on a large screen
             - check that the IME pushes the bubbles up
	       so they are not overlapping
             - check that IME on phone portrait & landscape
               works as it did before
Bug: 193911220
Change-Id: I3c93e472353cde5c938e33edf2d1ae7a4141db0e