sensorservice: Add proximity active callback APIs.

Bug: 175793106
Test: Tested using a temporary debug line in onProximityActiveLocked()
      and checking the counter going and up and down using the sensor
      test app plus the sensor_test CLI.
      Also tested willDisableAllSensors/didEnableAllSensors path by
      putting the service into restricted mode at various times.

Change-Id: Ic44d29f8f9de681cc6abaf722d6dca5266e358a3