Add gNMI device state subscriber

Change-Id: I20cb5e130f4e416bf8678aab2e5268faf24ad06b
diff --git a/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java b/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java
index f4964ed..b0e0071 100644
--- a/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java
+++ b/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java
@@ -17,6 +17,7 @@
 package org.onosproject.gnmi.api;
 
 import com.google.common.annotations.Beta;
+import org.onosproject.event.ListenerService;
 import org.onosproject.grpc.api.GrpcClientController;
 
 /**
@@ -24,5 +25,6 @@
  */
 @Beta
 public interface GnmiController
-        extends GrpcClientController<GnmiClientKey, GnmiClient> {
+        extends GrpcClientController<GnmiClientKey, GnmiClient>,
+        ListenerService<GnmiEvent, GnmiEventListener> {
 }