Add a new bubble at limit while expanded

Handles adding a new bubble and removing an old bubble when the bar
is expanded and at the limit.

Demo when bar is on right: http://recall/-/bJtug1HhvXkkeA4MQvIaiP/dxhFKrctdR5I2F6Pvho6u8
Demo when bar is on left: http://recall/-/bJtug1HhvXkkeA4MQvIaiP/er4ZGQfg8OKHZ2aTi9OJ9N

Flag: com.android.wm.shell.enable_bubble_bar
Test: demos
Test: atest BubbleViewAnimatorTest
Fixes: 345795791
Change-Id: I4e75d61c8afdb81340823a1d77e55b15e3fd6bc0