[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