[Android] Fix crash issue in Android extension system 

    On Android, the extension objects and instances are deleted explicitly
    by JNI invokation from Java side. However, both extension objects and
    instances are designed to be owned by XWalkExtensionServer, it means
    that only XWalkExtensionServer is allowed to delete them, otherwise a
    pointer would be deleted twice and cause crash issue.
    
    This fix is to address the above issue and make sure the extension
    objects and instances are owned by XWalkExtensionServer. Moreover, the
    extension objects on native side will be alive in the whole process
    lifecycle and reused by Java-side if resuming back.
    
    BUG=https://crosswalk-project.org/jira/browse/XWALK-821