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/transfer/TransferStatus.java

@@ -39,7 +39,7 @@ public enum TransferStatus {
 
   private final String message;
 
-  private TransferStatus(String message) {
+  TransferStatus(String message) {
     this.message = message;
   }