Fix bubble update animation

Fix bubble update animation when the updated bubble is the only
bubble in the bar.

Flag: com.android.wm.shell.enable_bubble_bar
Fixes: 352119655
Test: Manual:
       - Create 1 bubble in the bubble bar
       - Send an update for that bubble
       - Observe bubble bar is animated correctly
Change-Id: If569810dacf0ef77147a822c53ddebd09e35fe15