Allow Bubble overflow to be opened without other active bubbles

Currently you can only access the overflow if you have bubbles in
the stack. There are a bunch of explorations that show accessing
the overflow without the stack (e.g. from taskbar, from people area).

- BubbleData holds onto BubbleOverflow now
- When last bubble is removed, if showing in taskbar/already expanded
  & has overflow bubbs, select the overflow.
- No longer null out BubbleStackView when no more bubbles, this
  allows overflow to continue to show
- Added tests for overflow behaviour in BubblesTest

Test: atest SystemUITests
Test: manua 1) - produce a bunch of bubbles
               - expand the stack, dismiss each bubble
               => when you dismiss the last one, overflow selected
               - collapse the stack
               => overflow goes away
            2) - have taskbar cl / prototype
               - have bubbles in the overflow but no active bubbles
               => observe that taskbar shows the overflow (i.e. sysui
                 renders the stack in that spot)
Bug: 171322463
Bug: 167413172
Change-Id: I36b9cedc492ace59105dba58c89f9ef92ff4b9e8