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

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" />
Delegate making WebViewClient and ChromeClient to webview engine.

Revert the change of webview preference name.
Merge pull request #3 from huningxin/pluggable_webview

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