Fix #561 - Only collect StumblerBundle instances if we've got valid NMEA data

This adds an NMEA listener to the MainActivity and the Reporter.

An incoming NMEA data stream is a pretty good indicator that we're using a real GPS.

The MainActivity has stubs so that we can add visual feedback that NMEA data is being read.

The Reporter checks that we've received GGA and GSV sentences within the last 60 seconds or else StumblerBundle instances are not created.

I also did some minor code cleanup along the way.