fix [2476230] sensor battery stats could get out of sync if an error occurs

Fixed a few problems with the SensorService:
- a race condition when talking to the BatteryStatService
- only report changes to BatteryStatService when there are no errors
(ie: when a change actually happens)
- tell the BatteryStatService when a sensor is deactivated because its
client died
- rewrite enableSensor() so it's readable
- implement dump() so dumpsys will display some infos about active sensors
- recompute the delay properly when sensors are added/removed