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: I7db1d977276cbdd3446827cb3f3c96343b350e2c