The HTTP specification states that all clients are supposed to sent User-Agent headers. It however, does not state that they should identify the client in the manner a server wishes to. Android therefore complies with the specification, and there is not a lot you can do about it.
The best you could do is to use the setRequestProperty() method to attempt to get a preferable user-agent value in the request. There are no guarantees that it would work, but it is likely. The method needs to be called in the following manner:
connection.setRequestProperty("User-Agent","<em>USER-AGENT VALUE</em>");
Edit
Android uses WebKit. If you want to set WebKit's user-agent string, you'll either have to use a static value, or read the user-agent string from WebKit. I haven't attempted this, but the WebKit user-agent string is available in Android via the getUserAgentString() method of the WebSettings class.