Move ActivityView into BubbleExpandedView

This is initial work that needs to come before moving bubbles to the
bottom of the screen and enabling horizontal swiping.

* Moves ActivityView out of BubbleView and into BubblExpandedView
* New 'bubble' object that wraps the notification entry and the two
  bubble views (icon + expanded), this is utilized by BubbleStackView
* BubbleData is the knower of bubbles and used by BubbleController &
  BubbleStackView as source of truth

Bug: 123543995
Test: atest BubbleControllerTest (existing tests all pass)
Change-Id: I710d908e78ed2aef6a0e482b70c21fa0640d250c