Fix for Kryo related issue crossing OSGi bundle boundaries.

Change-Id: I121dfe360de14a5b4760e5d2fd8cb2db93e0be63
diff --git a/core/api/src/main/java/org/onlab/onos/net/provider/ProviderId.java b/core/api/src/main/java/org/onlab/onos/net/provider/ProviderId.java
index 8d3d571..725748a 100644
--- a/core/api/src/main/java/org/onlab/onos/net/provider/ProviderId.java
+++ b/core/api/src/main/java/org/onlab/onos/net/provider/ProviderId.java
@@ -12,12 +12,6 @@
     private final String scheme;
     private final String id;
 
-    // Default constructor for serialization
-    protected ProviderId() {
-        scheme = null;
-        id = null;
-    }
-
     /**
      * Creates a new provider identifier from the specified string.
      * The providers are expected to follow the reverse DNS convention, e.g.
@@ -40,6 +34,15 @@
         return scheme;
     }
 
+    /**
+     * Returns the device URI scheme specific id portion.
+     *
+     * @return id
+     */
+    public String id() {
+        return id;
+    }
+
     @Override
     public int hashCode() {
         return Objects.hash(scheme, id);