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/flows/domain/DomainTransferApproveFlow.java

@@ -207,9 +207,7 @@ public final class DomainTransferApproveFlow implements TransactionalFlow {
         autorenewEvent,
         gainingClientPollMessage,
         gainingClientAutorenewPollMessage);
-    if (billingEvent.isPresent()) {
-      entitiesToSave.add(billingEvent.get());
-    }
+    billingEvent.ifPresent(entitiesToSave::add);
     ofy().save().entities(entitiesToSave.build());
     // Delete the billing event and poll messages that were written in case the transfer would have
     // been implicitly server approved.