"p" prefix in Android linter

Summary:
Added a "p" prefix for member variables in private classes. Also fixed a few bugs:
- License linter had a weirded-up message
- Check for "R" class name was not working properly
- Generated file linter was not checking the correct path if `arc lint` was executed from a subfolder of the project

Test Plan: Run `arc lint` on an Android project with private classes.

Reviewers: mbishop

Reviewed By: mbishop

Differential Revision: https://devportal.imobile3.com/D4257