BubbleData [2/n]: adds Listener interface

This change introduces an interface which BubbleData will use to
report changes to the state of bubbles on-screen. This will allow
BubbleData to implement and propagate ordering and grouping changes.

Bug: 123542488
Test: atest BubbleControllerTest
Change-Id: I35708c45e23fed4369605eecd986cdff96e23d34