Clean Up and Flesh Out Mixpanel Implementation

This pull request is a parallel pull request to this one https://github.com/wordpress-mobile/WordPress-iOS/pull/1546 we did in the iOS Project. Like that pull request we wanted to clean up our stats implementation in the project so it wasn't so tied to Mixpanel but on top of that we wanted to flesh out our Android stats tracking capabilities as they were sorely lacking in comparison to the iOS Project.