Fix race in AppOpsManager client library

If the service stays down for more than 10 seconds, client threads
will time out attempting to obtain a reference, and never release
the mutex.  This causes deadlock on the next attempt to speak
with the AppOpsManager.

Test: System without AppOpsManager service no longer hangs in camera
      code.

Change-Id: I5a8ac2f0f35345f72c3d2a45b9d1fc5a78d28a51