Fix a possible SensorService deadlock.

 If the destructor of SensorEventConnection gets called when the main
 sendEvents loop of SensorService is executing it may result in a deadlock.
 The loop promotes each connection to a strong_pointer, calls sendEvents
 and cleans up the connection if necessary. It is possible that the sp's
 destructor may delete SensorEventConnection which will call the dtor
 ~SensorEventConnection(). This dtor again needs SensorService mLock to
 execute which may result in a deadlock.

Bug: 17617897
Change-Id: I76c244dbe85fadb591c0bd1a9a5eb01d93f56505