Bug fix and documentation clarification

Clarify definition of rotation angles used in
SensorManager.getOrientation and SensorManager.getAngleChange.

Clarify the condition of a free-fall in SensorManager.getRotation, and
added code to guard against such condition.

Fix a typo that causes confusion in the definition of
TYPE_ORIENTATION data in SensorEvent.

Change-Id: I614d981ec7b1b52c5ec59d5b40b84cd600df65e7
See: b/21337218, b/21337552, b/21337606.