Remove all imports of 'com.github.mobile.R.*'

This keeps Android Studio and Lint happier.

Closes #628
I was going to be a total nit about this. But, was not exactly sure. So, I let them in.
It's good you mention it @raziel23x .
For the sake of clean commit history, then I'm going to revert this.
@jonanp could you please move the non-related R string addition to a separate commit instead?