Cap the width of the bubble bar when collapsed

When the bubble bar is collapsed we now show at most 2 bubbles.

Fixes: 295020145
Test: Manual
      - Add 1 bubble to the bubble bar
      - Collapse bubble bar
      -- Observe that only that bubble is visible
      - Expand the bubble bar
      -- Observe that both that bubble and the overflow are visible
      - Add another bubble
      -- Observe that 2 bubbles are visible when collapsed
      - Add another bubble
      -- Observe that only the first 2 bubbles are visible when collapsed
Change-Id: I5f534f53eefe9a920b8b3258813d67d021c5f2a8