Fix NPE from freeing expanded view while expanding.

There was a potential NPE from a notification's expanded view being
freed while the notification was expanding. We fix this by by forcing
the view change when the transformation would involve a removed view.

Bug: 159510709
Test: Reproduce steps in bug, no crash
Change-Id: Ib0e6f7c57f29d2bb0157601d190dd0bb5ed6add1