Yell if NDC callers are holding bad locks.

The current MountService design heavily depends on down-callers not
holding any locks, since the vast majority of events are unsolicited
and bubble up to the framework.

This simple API gives us an easy way to track down people calling
while holding a lock they shouldn't be.

Bug: 25443096
Change-Id: Ifcbda95f00d5be8c1b88d58ca67927e76c713c3e