Introduce the devices Mojo app

It's a small Mojo app that currently only hosts USB device
services. This CL adds the app along with an apptest suite
(run manually with mojo_runner mojo:devices_apptests).

The app is hooked up to Chrome to run in the browser process,
though there are currently no production code paths which
cause the app to launch.

BUG=498557

Review URL: https://codereview.chromium.org/1165223004

Cr-Commit-Position: refs/heads/master@{#333899}
25 files changed