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

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

java/google/registry/model/registrar/Registrar.java

@@ -133,7 +133,7 @@ public class Registrar extends ImmutableObject implements Buildable, Jsonifiable
      */
     private final Predicate<Long> ianaIdValidator;
 
-    private Type(Predicate<Long> ianaIdValidator) {
+    Type(Predicate<Long> ianaIdValidator) {
       this.ianaIdValidator = ianaIdValidator;
     }