PIP dismiss updates

* orders the target below the PIP surface
* adds scale relative to the dismiss target size when PIP is in
  the target

Test: manual - get a pip, drag it to dismiss => it appears on top of
               the circle and then animates away with it
             - get a pip, make it larger with 2 finger gesture,
               drag it to dismiss => it appears on top of the circle
               and fits nicely in the circle bounds
Bug: 183657564
Change-Id: I5c096c2b5f41a667df37b4c6668c4862a212c7e2