Fixed b/10512887.

This is an issue caused by multi-threading. If geofence provider service
is connected and disconnected immediately, the ServiceWatcher can return a
null service handle to the private thread used by GeofenceProxy, and
this can cause NPE and system crash.
This CL also fixed a hidden race conditions bug where mGeofenceHardware
is not synchronized between two threads.

Change-Id: I824642cd638fbb1e6799a5a1220b047ebc2556a1