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/export/SyncGroupMembersAction.java

@@ -72,7 +72,7 @@ public final class SyncGroupMembersAction implements Runnable {
     final int statusCode;
     final String message;
 
-    private Result(int statusCode, String message) {
+    Result(int statusCode, String message) {
       this.statusCode = statusCode;
       this.message = message;
     }