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

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;
     }
 

java/google/registry/model/registrar/RegistrarContact.java

@@ -84,7 +84,7 @@ public class RegistrarContact extends ImmutableObject implements Jsonifiable {
       return required;
     }
 
-    private Type(String display, boolean required) {
+    Type(String display, boolean required) {
       this.displayName = display;
       this.required = required;
     }