Improved the clipping of notification groups

Groups now correctly respect the clipBottomAmount and
animations work much better when swiping things away.

Also removed the group measuring logic when expanding,
Since we're now not shrinking the first notification anymore!

This also fixed a bug where a child could be invisible

Test: Add group, swipe children away
Bug: 32437839
Bug: 33203156
Change-Id: I255aa9695086e64eb10c7dccdc6122d8a8572bb5