Fix broken NSSL tests

I don't see a reason why we would constrain mBarState.addListener to
happen only once, because the whole point is that other classes listen
to those changes as well, and will subscribe unless we mock literally
everything.

Change-Id: Id6aed68c48fbc4f94f78fcd38b9fe0ec83d49c7e
Fixes: 115742174
Test: `atest NotificationStackScrollLayoutTest`