Clear mEnterTransitionCoordinator after content shows.

Fixes: 233975804

After content has been shown, we should clear the
mEnterTransitonCoordinator so that the exit doesn't
accidentally try to use the enter transition information
to remove Views.

Test: manually tested example from bug report
Change-Id: Ida7ca354cb75953d1d5ecffc159da0d7320d4349