Fix potential race issue with removing bubble

When the last bubble is removed while it is expanded, we first hide the
scrim and then remove the bubble views.
As it takes time for the scrim animation to run, it is possible that
another bubble is added between the scrim animation starting and
finishing. This puts the BubbleStackView in a bad state where there is a
selected bubble, but we have cleared that value from the stack view.
Making sure that we only clear the expanded/selected bubble if it has
not changed.

Bug: 353894869
Flag: EXEMPT bugfix
Test: have one bubble open from Messages, force stop the app, check that
  the bubble is removed
Test: artifically slow down the scrim animation, have one bubble from
  Messages open, force stop the app, trigger a new Messages bubble with
  incoming sms quickly before the scrim animation completes, verify that
  new bubble is shown and if clicked, it expands
Change-Id: I5593bf889a58b6edfe4c4785f76d72a3b3163877