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/tools/MutatingCommand.java

@@ -54,7 +54,7 @@ public abstract class MutatingCommand extends ConfirmingCommand implements Remot
   private static class EntityChange {
 
     /** The possible types of mutation that can be performed on an entity. */
-    public static enum ChangeType {
+    public enum ChangeType {
       CREATE, DELETE, UPDATE;
 
       /** Return the ChangeType corresponding to the given combination of version existences. */