If we're adding / removing a bubble, don't forget to add the overflow

One case I missed was a bubble being added/removed AND the overflow
showing... this isn't the nicest, but when the add/remove animation
is done, animate the overflow in.

Flag: com.android.wm.shell.enable_bubble_bar
Fixes: 358304168
Test: manual - don't have the overflow showing, add a 6th bubble,
               observe that the overflow appears
Change-Id: I1a9cec3970f5535e935f3404fa6dbd6de2f69fa5