wait for sensor service before starting WMS

- Starting sensor service in separate thread led into occasional 1 sec
  blocking of WMS inside PhoneWindowManager to get sensor service.
- Change startSensorService into blocking call and call it from
  separate thread using SystemServerInitThreadPool.
- This does not improve best case boot-up time but fixes occasional 1 sec delay
  which is happening in 10 to 20% rate. So this is potential 100 to 200ms saving for many runs.

bug: 34846045
Test: multiple reboots.
Change-Id: Ia08fa3284aed5e576acaac6fbfd74b9db9f7d63c