Squid: Avoid cycle between java packages

Changed EventListenerRegistry (impl) to EventListenerRegistryService
(interface)
(forced by changes in http://review.motechproject.org/#/c/3279/)

https://trello.com/c/A0rrIWb0

Change-Id: Icfbbcf89be401b7db5f6654affe659889c2a71e1