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/NonEncodableDiscreteResources.java b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/NonEncodableDiscreteResources.java
index c285ae5..7b991b4 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/NonEncodableDiscreteResources.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/NonEncodableDiscreteResources.java
@@ -34,11 +34,15 @@
         return new NonEncodableDiscreteResources();
     }
 
+    static NonEncodableDiscreteResources of(List<DiscreteResource> resources) {
+        return new NonEncodableDiscreteResources(resources);
+    }
+
     private NonEncodableDiscreteResources() {
         this.values = new LinkedHashSet<>();
     }
 
-    NonEncodableDiscreteResources(List<DiscreteResource> values) {
+    private NonEncodableDiscreteResources(List<DiscreteResource> values) {
         this.values = new LinkedHashSet<>(values);
     }