Fix NPE from onChildVisibilityChanged.

Removing the child whose visibility changed in
ViewGroup#OnChildVisibilityChanged causes a null pointer exception since
the parent is null immediately after.  This CL prevents that from
keeping the parent stored.

Bug: 117520801
Test: remove child in OnChildVisibilityChanged, observe no crash
Change-Id: Ifd20c2fcba9aee476a7714794a90c7ec9a0b3b84