Add Theme item to preference drawer

Summary:
Let's keep it in settings and remove the theme icon. This seems
to be the conventional practice.

Test Plan: i need guys with navigation bars to test this. Long story

Reviewers: synergy, anders3408, jriordan001, raymanfx, donald.munn, randall.rushing

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D242