[Android] Destroy the native resource of XWalkView

Each XWalkContnet object owned by XWalkView 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