Replace WebView with XWalkView for Cordova 3.6.3

Crosswalk-cordova will not support the option of --shared.
There are no super.setWebChromeClient and super.setWebViewClient function in Xwalk,
so keep align with upstream logic.
Replace WebView with XWalkView.
Fix test app crash when exit with back key, refer to gaochun's commit "
af2ab8bb217e4fe5e3fa96d805cabc6a02ec6e83".
Explicit activity.finish() in back button handling.