sensorservice: Add legacy sensors fusion.

This change allows devices with accelerometer & magnetometer but
without gyroscope to still get virtual sensors such as Gravity,
Linear Acceleration and Rotation Vector.

Change-Id: Ibb90b70845c766ab52c843557446e34649a7d6d9

Conflicts:
	services/sensorservice/SensorService.cpp
native: add flag to disable legacy sensors fusion

Causes problems on all p31xx devices.
http://pastebin.com/PabjFVWP

Change-Id: Iac9a913e0984d7e9e419772215d3fb3d1a0cb5e8