Lint fixes

I fixed some of the issues that appear in Android Studio after running Analyze -> Inspect Code. The majority of these fixes are very simple, and I just used the automatic fix option in Android Studio. I've also removed all dead code that's currently in the app.

There are still tons of issues remaining. Many of them are more complex than the code I'm changing here, so it would be good if someone with more knowledge of the F-Droid code could have a look at the ./gradlew lint and Inspect Code output.