[sensorservice] Init batterystats service when needed

This avoid race condition at parallel services start up that
may results in sensor power usage not being properly recorded.

Test: check all sensors, they still work
Bug: 33199244
Bug: 33623300

Change-Id: I48946667db54fc72d9be6c13b981b44d1bab88c2
(cherry picked from commit 0a031596)