Add Android Omaha client architecture overview and new owners

This CL aims to describe how the Omaha client code works on Android,
since it is a bit different than our other platforms. This is done
as a README.md file in the main directory.

The document describes the core flow, triggering, requests and
responses, scheduling, class structure, and testing.

This also adds new users to the OWNERS file in the omaha client
directory.

BUG=806040

Change-Id: Ib20b65d00d4bf8489067a2a0595b8d78dafc32bd
Reviewed-on: https://chromium-review.googlesource.com/991057Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547737}

chrome/android/java/src/org/chromium/chrome/browser/omaha/OWNERS

 nyquist@chromium.org
+sorin@chromium.org
+waffles@chromium.org