Fix dismiss ciricle being behind expanded view

- Need to give it an elevation
- Need to make sure to bring the bubble container
  in front of it so that they appear on top

Test: manual - have at least 2 bubbles, expand the stack, drag
               the bubble that is not expanded to the dismiss view
             => observe that the dismiss view is above the expanded
                view & below the bubble
Bug: 190478161
Change-Id: Ifbce49211b2342c3e197168af38f4c2ca9a1904b