SensorService flush fixes.

 i) Fix a possible race condition when adding a pending flush connection
 to mActiveSensors data structure.
 ii) Invalidate flush_complete_events as soon as they are consumed by a
 connection so that they are not used by other connections which are
 registered for the same sensor.

Change-Id: I1491bea8c2081c4aab1feacc60add13b7aa26bb7