Automatic prep of library projects for unit tests

Summary:
arc unit now performs 'android update' on a project's library projects, if any.  No more manual prep!

WIP

Test Plan: run 'arc unit' on a project with library projects (like THD) and watch the prep taking place before your eyes

Reviewers: mbishop, jmillard

Reviewed By: jmillard

Differential Revision: https://devportal.imobile3.com/D4624