Check that a bubble exists before reordering it

This avoids re-adding bubbles that have already been removed
after dismissing multiple bubbles at once.

Fixes: 291738088
Test: Manual
      - Add 2 bubbles A and B to the bubble bar
      - Expand the bubble bar
      - Tap 3 dot menu and dismiss
      - Repeat for the next bubble
      - Observe that bubble bar is gone
      - Add a new bubble C
      - Observe that the bubble bar contains only bubble C
Change-Id: Id53811a103d245647489ab8d442cbb2004e0e700