SystemServer: Move sensor init to a new thread.

Move Sensor init to a separate thread so that it doesn't block the
critical booting path.

Initializing Sensors in separate thread has been seen to improve boot
time on customer devices with low-tier / ult chipsets(less cpu cores)

Change-Id: I5c46a5805c04ffe551019b4683523e3bf70274a2
Merge "SystemServer: Move sensor init to a new thread." into kitkat3