[sensorservice] Fix on-change sensor send latest value on enable

This CL correct the behavior of sensorservice so that the latest
value of on-change sensor is send to client when enabling sensor.

The errornous behavior before is that sensorservice send the oldest
value in local cache.

Bug: 28480791
Change-Id: I7e0b8e52fa0be62ba95910789cff1fe0d4fbb03f