Exclude generated fields from ImmutableClass check

- Eclipse sometimes auto-generates switch table, which fails the immutability unit tests.
  Ignore field name starting with _ or $, which tends to be used for auto-generated names.

----
Failed tests:
  IpAddressTest.testImmutable:41
Expected: is "a properly defined immutable class"
    but : was "a field named '$SWITCH_TABLE$org$onlab$packet$IpAddress$Version' that is not final"
  IpPrefixTest.testImmutable:39
Expected: is "a properly defined immutable class"
    but : was "a field named '$SWITCH_TABLE$org$onlab$packet$IpAddress$Version' that is not final"
----

Change-Id: Ibca5f61b9ca6b6006424a4288f1863b6e60ad484

@@ -52,7 +52,9 @@ public class ImmutableClassChecker {
 
         // class must have only final and private data members
         for (final Field field : clazz.getDeclaredFields()) {
-            if (field.getName().startsWith("__cobertura")) {
+            if (field.getName().startsWith("_") ||
+                field.getName().startsWith("$")) {
+                //  eclipse generated code may insert switch table - ignore
                 //  cobertura sticks these fields into classes - ignore them
                 continue;
             }