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

Crosswalk core library is changed and the extension mechanism/implementation
now is inside of core library. No need to call extension manager anymore.

Besides, onPause/onResume/onDestroy functions are called in CordovaWebView.
It's because we don't only pause/resume timers but also need to pause/resume
video playback, popup dialog and others. All of these are handled in above
functions.