Update bubble stash test for transient taskbar

Include unit test to check that view properties are set to expected
values at the end of an unstash animation.
Add tests to check that alpha for bubbles and background changes
separately during unstash and stash animations.

Bug: 345488489
Test: TransientBubbleStashControllerTest
Flag: com.android.wm.shell.enable_bubble_bar
Change-Id: Ifd10d4524128d42d23fe67c0eff2c4ae16d3b27b