Create a drag controller for expanded bubble bar

Tracks touch events on BubbleBarHandleView.
Moves the BubbleBarExpandedView around based on touch events.
When finger is released, snaps back to the initial position.

Bug: 283991264
Test: drag from the handle
  - observe that the view follows the finger
  - release touch, observe that the view snaps back

Change-Id: I16bc97270195dbdb753ac30d091a93c63ce6628d