Navigating to http://whatsmyuseragent.com/ shows me my stock Android browser on my Galaxy Nexus running 4.2.1 has the user agent
Mozilla/5.0 (X11; Linux x86_
Your'e not getting android in the UA because you're using Desktop View. There's seemingly no way to identify that this is a mobile device in that mode. Once you'll switch to normal view you'll see the right UA.
android