Animate alpha for bubbles and background (3/n)

Use separate timings for bubble icons and bubble bar background when
stashing and unstashing.
Follows alpha timings applied to taskbar background and icons.

Bug: 345488489
Test: TransientBubbleStashControllerTest
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: I485f6346539cb6c8ea6dd4d15f25a6421021fec1