Align bubble bar stash anim with taskbar (1/n)

Only scale bubble bar background during stash and unstash animation.
Follows the logic used for taskbar. We scale only the background and
clip the icons separately.

Bug: 345488489
Test: PersistentBubbleStashControllerTest
Test: TransientBubbleStashControllerTest
Test: BubbleBarViewAnimatorTest
Test: stash and unstash bubble bar in app by swiping up from taskbar
Test: expand and collapse bubble bar in app by swiping up on bar
Test: expand and collapse bubble bar on home screen by tapping on it

Flag: com.android.wm.shell.enable_bubble_bar
Change-Id: Ifc7922c444f2179fc49643424815e5e7dde519cc