AI 143244: am: CL 142801 am: CL 142799 Ensure thread exit resets mThread to...

AI 143244: am: CL 142801 am: CL 142799 Ensure thread exit resets mThread to null (synchronized with sListeners)
  If sensors_data_poll failed the sensor thread would exit leaving
  mThread non-null, hence SensorManager would never again attempt
  to start the thread because it assumes that it's still running,
  compromising future sensor notifications.
  Original author: andih
  Merged from: //branches/cupcake/...
  Original author: android-build
  Merged from: //branches/donutburger/...

Automated import of CL 143244