Make correct webview client and chrome client for specific webview engin...

...e.

It changes the webview preference naming from full name to prefix, since the
prefix is also used to construct the name of WebView, WebViewClient and
ChromeClient.

For example, for Crosswalk webview, config.xml contains:
<preference name="webView" value="org.apache.cordova.engine.crosswalk.XWalkCordova" />