Jelly: Don't claim to be a simple WebView

If the user agent string includes "wv", then some websites indentify
the browser as a simple WebView and stop working properly (e.g. Google
OAuth 2.0 can't be used). Fix this removing "wv" from the user agent.
Note that this commit also changes the way the desktop user agent
is generated so that a single regular expression is used.

REGRESSION-519

Change-Id: Id138e5b38c6de7476084d7717c59f11ebe7d8178