Fix flake in BubblesTest

This change updates BubblesTest to wait for any physics animations
to finish in the teardown of the test.

ABTD: https://android-build.corp.google.com/abtd/run/L94200030005122446/

Flag: TEST_ONLY
Fixes: 351977103
Test: abtd link above

Change-Id: I48e29d0e5e5197ff4f766808e672a0bcf5af374a
2 files changed