Changed member variables in private classes to not use "m" prefix

Summary: Changed the custom naming convention linter to not use the "m" prefix for member variables on private classes.

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

Reviewers: mbishop

Reviewed By: mbishop

CC: jstarke

Differential Revision: https://devportal.imobile3.com/D4147