An attempt to unregister a handler that's no longer registered should not cause an assertion.

There are edge cases in which a handler attempts to unregister itself while on another thread a message fails to be delivered to that very handler and causes automatic unregistration. In this case the handler's good cleanup intentions are thwarted by the CHECK.

Change-Id: I4e41b5e7b619159ecce4856c15cccca031a28b5b
related-to-bug: 3101247
QA-impact: no(!!!) risk