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 
Documentation of properties should be a multi-line /** comment. Naming is a bit misleading here, it's not a flagType, it's a flagTypeManager. similar for entityQuery below, I'd add factory there because it's a factory to create an entity query.