Untested fix to allow Ethernet connectivity

At present, CheapCast only works if the Android device is using wi-fi for its network connectivity. This means that CheapCast can't be used on devices such as an Ouya if they're plugged into an Ethernet connection.

This PR theoretically fixes this. Unfortunately, I can't figure out how to compile CheapCast to actually test it, but I've tried the heuristic in the PR in a standalone app and it seems to detect the right address on both my phone and my Ouya. (It probably won't work properly on the ATV1200 I have floating around, since that has both Ethernet and wi-fi active at once, but one problem at a time.)

This may require some massaging before it's releasable, but the approach is sound. This would fix issue #8.