make Filter object immutable

Change-Id: I73063506cab0200449197245e8be85d3868d5327
diff --git a/apps/l3vpn/src/main/java/org/onosproject/l3vpn/netl3vpn/impl/NetL3VpnManager.java b/apps/l3vpn/src/main/java/org/onosproject/l3vpn/netl3vpn/impl/NetL3VpnManager.java
index cf9a405..cc0d51e 100644
--- a/apps/l3vpn/src/main/java/org/onosproject/l3vpn/netl3vpn/impl/NetL3VpnManager.java
+++ b/apps/l3vpn/src/main/java/org/onosproject/l3vpn/netl3vpn/impl/NetL3VpnManager.java
@@ -1025,7 +1025,7 @@
         public void processItems(List<DynamicConfigEvent> events) {
             for (DynamicConfigEvent event : events) {
                 checkNotNull(event, EVENT_NULL);
-                Filter filter = new Filter();
+                Filter filter = Filter.builder().build();
                 DataNode node;
                 try {
                     node = configService.readNode(event.subject(), filter);