Added support for GVR controllers

Currently uses a different instance of GvrApi than the VrShell, but it's not
clear if that's something that's going to cause a problem. Works well enough for
the time being.

BUG=389343

Review-Url: https://codereview.chromium.org/2351573002
Cr-Commit-Position: refs/heads/master@{#420240}
8 files changed