Make Hello flows use the flow "now" time

This cleans up some of the tests, and helps with
future injection CLs.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124208164

java/google/registry/flows/session/HelloFlow.java

@@ -22,6 +22,6 @@ import google.registry.model.eppoutput.Greeting;
 public class HelloFlow extends Flow {
   @Override
   public EppOutput run() {
-    return EppOutput.create(new Greeting());
+    return EppOutput.create(Greeting.create(now));
   }
 }