Bubbles handle user switch

 - Removes bubbles on user switch
 - Saves notification keys for active bubbles
 - Restores bubbles on switch back
 - Flyout text is suppressed when bubble is restored

Cleanups:
  Removes setRemoved() from Bubble (no effect)

Bug: 131609280
Test: manually, add bubbles, add second user, switch to
    second user, switch back to primary
Change-Id: I0f2c6a2f2a46e0c812291e38aa79497a8d36592e