Fix wrong assertions

Before, CollapseBubbleViaBackTest assumes bubble layer should exist
at the end of transition. However, the surface may be gone because
the surface of bubble app may be removed.

This CL adjusts assertions to fix the test

Fixes: 426454970
Test: atest CollapseBubbleViaBackTest
Test: atest DismissExpandedBubbleViaBubbleViewTest
Flag: TEST_ONLY
Change-Id: Id7553ed7b140d6e3ddf4293252b4b15be9e3e7fc