Simplify bubble z ordering and fix various issues

Issues this CL addresses:

1) When expanding / collapsing the overflow was layered above other
   bubbles incorrectly, it should always be at the bottom.
2) When expanded, the bubbles has inconsistently applied z
   translation, i.e. only the top two had z translation and the
   rest would not
3) Per spec, when expanded, bubbles shouldn't have elevation. (*)
4) When dragging out a bubble, it wouldn't be layered above all
   the other bubbles so if you dragged it on top of them it might
   get layered underneath

Additionally, this CL simplifies the code path for applying the
z translation instead of splitting it across multiple methods.

(*) It's actually pretty tricky to have bubbles have 0 translation
while expanded, because they need to have a specific z-ordering
for the collapse / expand animation and it's pretty obvious when
that shadow is animated in/out if you do it at the end of the
animations. Instead of setting the translation to 0, I give them a
minimal translation while expanded to ensure proper ordering. This
is basically imperceptible  because they are already on a scrim.


Test: manual - expand an collapse the stack while observing the
               overflow
             - expand and drag a bubble on top of the other bubbles,
               observe that it is always on top
             - drag a bubble and have it snap back, observe the
               shadow doesn't flicker
             - drag the bubbles around and observe the order is
               correct
Bug: 337933985
Change-Id: I0eb23dc8e89da6f576cb6981832273346fa0657f

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