sensorservice: add fusion sensors if all events are supported

- Fusion sensors depend on gyro, mag field, and accelerometer
  events in order to emulate these sensors.  Don't create
  them unless ALL of the real hardware devices required exist.

Change-Id: I1bce167a52c2dfc56a07450f858b01b78c39b7fe
Merge pull request #8 from LiquidSmooth/kitkat

sensorservice: add fusion sensors if all events are supported