SensorService fixes.

   i) Initialize sensor_event.flags to zero or wake_up flag before sending dropped flush_complete_events. This will avoid unnecessary acks. 
Currently it is not set to anything which may result in garbage values for flags field.
   ii) Change the dump format of SensorEventConnection.
   iii) Add const to method signatures wherever applicable.

Bug id: https://b2.corp.google.com/issues/17498626

Change-Id: Ia736ffb01c366540aab73cdcf596ce852c052593