Fix an issue where onAllBubblesAnimatedOut could be called incorrectly

There is a check for BubbleStackView having children before this
is called, however, BubbleData might have bubbles before the views
have been updated -- add a hasBubbles check which checks data.

Flag: EXEMPT very safe bug fix
Test: atest BubbleControllerTest
Test: manual - spam bubbles while dismissing them
Bug: 420487074 (not cause of this bug but found while trying to repro)
Change-Id: I354d9199f6f897bab5e889fb73c67c295f16a814