Fixed flaky proximity alert tests.

Fixed b/10667162. It's a regression in JB-MR2.
This is caused by race conditions in multiple threads and processes.
Because addProximityAlert returns before geofences are being monitored,
it's possible that the mock location inside the geofence can't be received by
GeofenceManager if it's sent immediately after addProximityAlert.

Change-Id: I7208bb288efe6bdf840a54b7e1544ebf96a45f2f