allow rotation-vector to have 4 components

- upadte documentation for rotation vector
- update method dealing with rotation vector to deal with 4 components
- virtual rotation-vector sensor reports all four components
- improve SensorManager documentation layout

Whent he 4-th component of the rotation-vector is present, we can save
a square-root when computing the quaternion or rotation matrix from it.

Change-Id: Ia84d278dd5f0909fab1c5ba050f8df2679e2c7c8