Fix NPE when mDecor is null on ActivityClientRecord

It seems like this case is handled in all other instances that mDecor is
accessed, so I can only assume that this should also check for null.

Bug: 36396248
Test: Compile and run VR application, crash no longer observed.
Change-Id: I53559bd6f1b9e14f93b8f3881ba5b52b45289f65