Remove Bubbles from the WM, only after all bubbles finish animating out.

This fixes the issue where the last bubble doesn't get an exit animation, saves memory, and is less unnecessarily risky when you have no bubbles.

Test: add and remove a lot of bubbles
Bug: 134514227
Change-Id: If3e3164e8a76a0155496b05e5ba8994d8da61c1a