Change SendEmailService to an instance field.

This allows us to inject it with Dagger and avoid using InjectRule to set it
in unit tests.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217571475

java/google/registry/util/SendEmailService.java

@@ -15,6 +15,8 @@
 package google.registry.util;
 
 import java.util.Properties;
+import javax.inject.Inject;
+import javax.inject.Singleton;
 import javax.mail.Message;
 import javax.mail.MessagingException;
 import javax.mail.Session;
@@ -22,8 +24,12 @@ import javax.mail.Transport;
 import javax.mail.internet.MimeMessage;
 
 /** Wrapper around javax.mail's Transport.send that can be mocked for testing purposes. */
+@Singleton
 public class SendEmailService {
 
+  @Inject
+  SendEmailService() {};
+
   /** Returns a new MimeMessage using default App Engine transport settings. */
   public Message createMessage() {
     return new MimeMessage(Session.getDefaultInstance(new Properties(), null));