Wait for expand animation to finish

When testing if task bounds are updated at the end of "back to rest
position" animation, wait for expand animation to finish.
Matches logic from other test cases.

Bug: 385707959
Bug: 385096726
Flag: EXEMPT fixing test
Test: atest WMShellMultivalentTestsOnDevice:com.android.wm.shell.bubbles.bar.BubbleBarAnimationHelperTest
Change-Id: I54c8ee9b8e7a4818a19f51cf8c9e86486bc50a88