Show dismiss target when dragging expanded view

Include DismissView in BubbleBarLayerView.
When drag starts, show it. When it ends, hide it.
Wire up a listener so that the new drag controller can notify layer
view.

Bug: 283991264
Test: tested following scenarios
  - drag a bubble and release it while finger in dismiss circle area,
    observe that bubble is dismissed
  - drag a bubble and release it outside dismiss circle area, observe
    that bubble snaps back to initial position
  - drag a bubble into dismiss area and out, release, observe that
    bubble snaps back to initial position

Change-Id: I58f870c304a6eb52b03f067a8a007e7aaae55e33