Use ExtensionRegistry instead of class name

Use ExtensionRegistry instead of class name

Use the ExtensionRegistry instead of checking for the main class
name of the Cite extension. - I'm not entirely sure if this is much
better. A hook would still be the better option. But at least Cite
could be refactored and the class name changed.

Bug: T89151
Change-Id: I35e5aa9955141b575de68a5be2c0d5b87585eb77