Flag service fixes

* Add arguments to service definition
* Remove unused class import
* Import new classes
* Add class properties
* Update class constructor
* Add return argument type
* Remove Drupal static calls
* Add return argument type

Issue #33 
I committed a fix for the member var names and docblocks in https://github.com/socketwench/flag-drupal8/commit/e9071a3be603c607dc896267e6cc344dd4e44549