Make the autocomplete matcher function into a service

The Autocomplete controller currently uses a private function to get the matches and return the keys and labels. This prevents it from being overridden is a Drupal 8 way. If the code is refactored to make the matcher function into a service (as exists in the Core implementation), and then let Realname override the core Drupal matcher service with its own, it will allow other modules to further override the realname matcher service is a Drupal 8 way.