Fixed a bug with not-gone notifications

Notifications with a GONE parent broke the whole scrolling model.
The shelf is now always placed at the end instead and the number
of GONE views are measured.

Test: Add a group with a single notification and see that nothing jumps
Bug: 32437839
Change-Id: I27a8c0b46eb8f329f23ad46c3995a19fb839caff
3 files changed