Switch debugAssertNotDisposed to be a static (#104772)

This reverts part of the change made in #103456 to expose a debug check for subclasses of ChangeNotifier to avoid code duplication. Instead of making debugAssertNotDisposed a public instance function, it is now a public static function. It makes it harder to call, slightly, but it means that everyone who implemented ChangeNotifier instead of extending it doesn't get broken.