Fix 'NetworkOnMainThreadException' error with AsyncTask in an SIP application

You will be provided with the source code of an SIP application. If using target API versions 11 and above (Honeycomb), the error message 'NetworkOnMainThreadException' is thrown in LogCat when attempting to register with an SIP server. Your task is to use AsyncTask to make sure all networking operations are performed in the background instead of on the main / UI thread. As a result, the error message should be solved and registration should occur successfully on API versions above 11. You are required to keep note of the files being modified in the source and will need to present this list of files on request.