Animate bubble removal from the bar

This change creates a BubbleAnimator class that handles animating
both new bubbles in and removed bubbles out.

Demo: http://recall/-/bJtug1HhvXkkeA4MQvIaiP/dMHGANplWjGC5HZT6MXyA4

Flag: com.android.wm.shell.enable_bubble_bar
Fixes: 345687290
Test: atest BubbleAnimatorTest
Test: see demo

Change-Id: Ie060495f4b1421d1c8af48a861cd3e12dd0adbe8