Fix fingerprint aidl race condition then thread is join

Even if the shared variable is atomic, it must be modified under the
mutex in order to correctly publish the modification to the waiting
thread.

There is a chance that mThread will miss both mIsDestructing
and notify event without a mutex.

Bug: 231737939
Test: manual
Change-Id: If83a94b766c816b9e4897acb1e89eacdce880c15