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/FlowComponent.java

@@ -122,7 +122,7 @@ public interface FlowComponent {
 
   /** Module to delegate injection of a desired {@link Flow}. */
   @Module
-  static class FlowComponentModule {
+  class FlowComponentModule {
     // WARNING: @FlowScope is intentionally omitted here so that we get a fresh Flow instance on
     // each call to Provider<Flow>.get(), to avoid Flow instance re-use upon transaction retries.
     // TODO(b/29874464): fix this in a cleaner way.