Refactor Cordova container to use the new API defined in Crosswalk.

Crosswalk defines the new embedding API which is public to external users.
Migrate Cordova container to use the new API. Due to some legacy issues,
there are still some remaining dependencies in Cordova on Crosswalk internal
classes, including XWalkSettings, XWalkClient, XWalkWebChromeClient and
XWalkHttpAuthHandler.
Next is to figure out how to remove above dependencies.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1375