Support hiding / showing the bubble overflow on BubbleStackView

Adds BubbleStackView#showOverflow which handles adding or removing
the bubble overflow view from the stack.

- BubbleOverflow always exists, but it's only added / shown when
  the flag is on and we have overflow contents
- Can't assume that we have an overflow all the time so need to
  be careful with counting views on the stack
- We "naturally" have an animation when views get added while
  expanded

Flag: ACONFIG com.android.wm.shell.enable_optional_bubble_overflow DEVELOPMENT
Test: manual  - enable the flag, have no bubbles in the overflow
              - add some bubbles, check that there's no overflow
              - dismiss a bubble
              => observe that the overflow appears
              - collapse / expand stack, overflow is still there
              - dismiss another bubble
              - tap on the overflow, add a bubble, tap on the overflow
                add the last bubble
              => observe the overflow disappears
              - turn off the flag and check that the overflow is
                always visible even when empty
Bug: 334175587
Change-Id: I9458827047230c4b0f4da624ff8fa83c41bf399f