Add facade object called UnifiedDiscreteResources

This is for ONOS-4281

Change-Id: I04649932fdb8983492878732a04c93a21b4cafe7
(cherry picked from commit ac6ff6e1984e0351938467be7677072888258c7d)
diff --git a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/DiscreteResources.java b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/DiscreteResources.java
index 8fc0bba..6b7da8e 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/DiscreteResources.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/DiscreteResources.java
@@ -32,7 +32,17 @@
      * @return a empty set.
      */
     static DiscreteResources empty() {
-        return NonEncodableDiscreteResources.empty();
+        return UnifiedDiscreteResources.empty();
+    }
+
+    /**
+     * Create an instace from the specified resources.
+     *
+     * @param resources resources
+     * @return instance
+     */
+    static DiscreteResources of(List<DiscreteResource> resources) {
+        return UnifiedDiscreteResources.of(resources);
     }
 
     /**