Animate the bubble bar on dismiss

When the last remaining bubble is dismissed by drag or from the menu
we now fade out the bubble bar.

Flag: com.android.wm.shell.enable_bubble_bar
Fixes: 347062801
Test: manual
       - create a single bubble in the bar
       - expand
       - dismiss by drag or from menu
       - observe animation
Change-Id: I86fcaf04c6f5949126744b81e0042b0e58318a13