Refactor the usage after the changes of extension mechanism in core libr...

...ary.

Crosswalk core library is changed and the extension mechanism/implementation
now is inside of core library. No need to call extension manager anymore.

Besides, calling onPause/onResume/onDestroy functions in CordovaWebView. This
is because we don't only pause timers but also need to pause video playback,
popup dialog and others. All of these are handled in above functions.