Add shadows back to bubble avatar

* Add shadows back -- icon lib is no longer adding shadows but
  bubbles should still have them for background protection.
* The shadow is active on all bubbles when:
  - the stack is being dragged around & bubbles trail behind it
  - the stack is expanded (the bubbles can be dragged to dismiss &
    can show on top of the expanded view or overflow)
* When the stack is resting, only the top two bubbles show a shadow
  & the bubbles below that animate their Z translation so the change
  is smooth.
* Also put the # of bubbles we show in the stack in a static int &
  updated usages

Test: visual - add 5 of the 'test bubbles' from the test app
               and notice the white backgrounds don't blend together
               due to the shadow & that they are distinct when
               placed on a white background
             - drag / fling the stack around & notice shadows show
               for all the bubbles while moving but once they come
               to rest you don't see a stack of shadows
Bug: 183657577
Change-Id: I0c273f4d0261db9eac1b49ef584fd62fbb67cc9a