Add the loadUrl implementation according to CordovaWebView interface.
Need to use org.xwalk.core.JavascriptInterface annotation

Otherwise the XWalkView.addJavascriptInterface() doesn't work.

See details at: https://crosswalk-project.org/jira/browse/XWALK-1615

It applies to Crosswalk-6 and later.
Merge pull request #1 from huningxin/master

Add CordovaWebView.loadURL(), and use correct JavascriptInterface annotation in XWalkExposedJsApi