[Android] Destroy the native resource of XWalkContent

Each XWalkContnet object has its own native counterpart, we need to
cleanup the native resource when an XWalkContent object is requested to
be destroyed, e.g. an activity is destroyed.

This fix is to add 'destroy' method to XWalkContent for this purpose.

BUG=https://github.com/crosswalk-project/crosswalk/issues/998