I'm wondering if this function could build wrong values for the parameter to pass to the `setPreferredNetworkType()` function. Possible value are at http://goo.gl/kVmLly and I'm a bit worried about building a wrong  and then pass it to the function.  

@EdgarChen Need your feedback here please. Take a look at the  `_getDefaultPreferredNetworkType()` function below please. Could this function build a wrong value to pass to the `setPreferredNetworkType()` function? Thanks.