Allowing heads-up notifications now on the lock screen

In order to isolate notifications properly from their group
and in order to allow them to visually reorder, we now allow
heads up notifications on the lock screen.

Change-Id: I1d04a92e66ae92ecee4f8ee23b218c2b167dadd4
Test: existing tests pass
Test: add heads up on lockscreen
Test: heads up group child on lockscreen
Bug: 32442500