Separate responsibility of MainActivity, fix strict mode violations

MainActivity is acting as both a view and a controller of the entire app, separating that responsibility to the new MainApp class.

I was seeing a rare crash rotating the screen an hitting the start/stop button quickly. This correlated with the instance count violation in the log, as reported from issue #7
I don't like seeing instance count violations, because I have no idea if they are contributing to app crashes.
https://github.com/mozilla/MozStumbler/issues/7
After this refactor, I no longer see this issue, nor can I get the app to crash rotating and hitting the scanning button.