Ensure a CompletedIntentUpdate subclass is the final state

Change-Id: Icecf8a45fad7cb80f3953dcfc081c344d3fa0adb
diff --git a/core/net/src/main/java/org/onosproject/net/intent/impl/CompletedIntentUpdate.java b/core/net/src/main/java/org/onosproject/net/intent/impl/CompletedIntentUpdate.java
index 60dc334..612ae1e 100644
--- a/core/net/src/main/java/org/onosproject/net/intent/impl/CompletedIntentUpdate.java
+++ b/core/net/src/main/java/org/onosproject/net/intent/impl/CompletedIntentUpdate.java
@@ -22,12 +22,12 @@
 /**
  * Represents a completed phase of processing an intent.
  */
-interface CompletedIntentUpdate extends IntentUpdate {
+abstract class CompletedIntentUpdate implements IntentUpdate {
 
     @Override
-    default Optional<IntentUpdate> execute() {
+    public final Optional<IntentUpdate> execute() {
         return Optional.empty();
     }
 
-    IntentData data();
+    public abstract IntentData data();
 }