Only notify bubbles if the notification panel expansion has changed

This callback is triggered for a couple of other changes, not just
for the notification panel expansion changing. This CL saves the
previous panel state and only triggers the bubble's callback if the
state was different.

Flag: None
Test: manual - observe logs that we're only notified when it actually
               changes
Bug: 323592544
Change-Id: Ic715e8d4774f8c0025cd1d943ed0bf3de129a129