Last week I came across the most annoying bug in Android where WebView URL mechanism is broken when we pass query string.
Fortunately an Android project member did provide a workaround for the issue, until it is fixed in Jelly bean update. However the work around given is in Java so, I have coverted the code to C# for developer targeting Xamarin! Hope it helps you.
Get the code from Gist here: https://gist.github.com/prashantvc/5013207