Divided accelerometer values on Android by gravity to match XNA and iOS.

XNA and iOS report accelerometer values in gravitational units, default value is (0; 1; 0).
Android reports values in SI units (m/s^2), default value is (0; 9.81; 0).
This PR makes [Android](http://developer.android.com/reference/android/hardware/SensorEvent.html#values) accelerometer values match [XNA](http://msdn.microsoft.com/en-us/library/windowsphone/develop/microsoft.devices.sensors.accelerometerreadingeventargs.aspx) and [iOS](https://developer.apple.com/library/ios/documentation/coremotion/reference/CMAccelerometerData_Class/Reference/Reference.html#//apple_ref/doc/c_ref/CMAcceleration).
Android apps that were using old values will be broken.