Fix NPE caused by config change on a destroyed activity

The config callback from view root may not know the state
of activity. That may trigger a stale callback, so
 - Restore the null check when getting mSizeConfigurations.
 - Clear the callback if the activity will be removed.

Bug: 192603207
Test: ActivityLifecycleTopResumedStateTests
Change-Id: I17ea8fc2ba994f630d5ad7baa3de298d90a32827