Support identification of APK channel distribution.

* For appstores and preload OEMs.
* The User-Agent will get updated if channel descriptor present.
* See comments for further information.
* A follow-up patch should enhance make-release to update manifest for APKs.

Change-Id: Iaf4b479821e9dadc32be5548c679440cb0f3510a
release 73

Change-Id: I39799edb957e9ad98ba121b05b215f714765d221
Merge "release 73"