Add animateAlpha to add/remove animations

Adding/removing notifications may push other group children beyond the
edge of the group container which should have an alpha effect applied as
it clips out.  So we add animateAlpha to add/remove to keep this
animation.

Bug: 150717795
Test: manual, post group, add notification, watch child clip out
animation

Change-Id: Id7e354934f1df86470114277b954f42ad09df562