Fix the spacing between bubble bar and expanded view

Store the bubble bar screen coordinates directly in BubblePositioner
and use that to calculate the bottom position of the expanded view.

Screenshot: https://screenshot.googleplex.com/5q9VNWW3TUP9hgy

Fixes: 290099589
Test: Manual
       - Add bubbles to the bubble bar
       - Expand the bubble bar
       - Observe that there is space between the bar and the expanded view

Change-Id: I6b8f35663bb544cc54b734fd6d34bf5593206fab