Fade bubbles when dismissing the last bubble

When the stack is expanded and the last bubble is being dismissed,
the bubbles now fade out as part of the collapse animation.

Demo: http://recall/-/bJtug1HhvXkkeA4MQvIaiP/fl3WS1vhKAwKFJDu05uQ6V

Fixes: 311485533
Test: Manual
      - Add a bubble and expand the stack
      - Dismiss the bubble either from the manage menu or by dradding it
      - Observe that bubbles fade out during the collapse animation
Change-Id: Ieecfbfe27bddde72355aaf54b14eb9abeb1e5e06