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/Command.java

@@ -26,5 +26,5 @@ public interface Command {
    * <p>Just implementing this is sufficient to use the remote api; {@link RegistryTool} will
    * install it as needed.
    */
-  public interface RemoteApiCommand extends Command {}
+  interface RemoteApiCommand extends Command {}
 }