Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

javatests/google/registry/util/ComparingInvocationHandlerTest.java

@@ -33,7 +33,7 @@ public class ComparingInvocationHandlerTest {
 
   static class Dummy {}
 
-  static interface MyInterface {
+  interface MyInterface {
     String func(int a, String b);
 
     Dummy func();