Enable checkstyle rule for JLS modifier order

JLS specifies a preferred order for modifiers in
declarations.  Enabled this rule and fixed the
problems that it encountered.

Change-Id: I5e8fa3d29f785ced4c61fbb8c7c8b168289f06e2
diff --git a/src/main/java/net/onrc/onos/core/util/FlowId.java b/src/main/java/net/onrc/onos/core/util/FlowId.java
index 5eace27..f57701e 100644
--- a/src/main/java/net/onrc/onos/core/util/FlowId.java
+++ b/src/main/java/net/onrc/onos/core/util/FlowId.java
@@ -16,7 +16,7 @@
 @JsonDeserialize(using = FlowIdDeserializer.class)
 @JsonSerialize(using = FlowIdSerializer.class)
 public final class FlowId implements Comparable<FlowId> {
-    private final static long INVALID = -1;
+    private static final long INVALID = -1;
     private final long value;
 
     /**