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/ImmutableObject.java

@@ -52,7 +52,7 @@ public abstract class ImmutableObject implements Cloneable {
   @Documented
   @Retention(RUNTIME)
   @Target(FIELD)
-  public static @interface DoNotHydrate {}
+  public @interface DoNotHydrate {}
 
   @Ignore
   @XmlTransient