Add a signal for the contents of the overflow going from empty to not

- When bubbles are added / removed from the overflow, check if
  the empty state has changed and if so create a change event
- Adds a method to the view listener for this event

Note this CL doesn't actually do anything with the views, this will
be done in a follow up.

Test: atest BubbleDataTest
Bug: 334175587
Change-Id: I231040383f10ceed49576f29bd3660e23905a4ba