Remove event sink when app is deactivated.

Change-Id: I4c0b2b39041a8db8a2bceba8e5e615edec90ed88
diff --git a/app/src/main/java/org/opencord/olt/impl/Olt.java b/app/src/main/java/org/opencord/olt/impl/Olt.java
index 33206e9..9717bad 100644
--- a/app/src/main/java/org/opencord/olt/impl/Olt.java
+++ b/app/src/main/java/org/opencord/olt/impl/Olt.java
@@ -205,6 +205,7 @@
         deviceService.removeListener(deviceListener);
         networkConfig.removeListener(configListener);
         networkConfig.unregisterConfigFactory(configFactory);
+        eventDispatcher.removeSink(AccessDeviceEvent.class);
         log.info("Stopped");
     }