Support optional bubble overflow in bubble bar

This is similar to the animations that add / remove a bubble at the
same time -- the overflow is generally added when a bubble is removed.
The overflow is generally removed when a bubble is added (i.e. user
promotes a bubble out of the overflow).

There are a couple of additional cases:
- when bubbles are first added to the bar -- if there were saved
  bubbles in the overflow, the view should be added
- an app could cancel its bubbles / remove its shortcuts and not have
  any in the stack but could have some in the overflow & it could
  become empty without an addition.

Flag: com.android.wm.shell.enable_optional_bubble_overflow
Flag: com.android.wm.shell.enable_bubble_bar
Test: manual - add bubbles to the bubble bar for first time
             => observe there is no overflow
             - dismiss a bubble
             => observe the overflow is added, tap on it, tap on the
                bubble in it
             => observe that bubble is added & the overflow disappears
             - dismiss all the bubbles
             - add a bubble
             => observe the overflow is there & has the previously
                dismissed bubbles
             - cancel all the bubbles that are in the overflow via
               adb
             => observe the overflow is remvoed
Bug: 334175587
Change-Id: I2b6e855e65520b4b2b1fde7757d46f00a468b4a6