Fix ApiFutures.addCallback() (#3886)

ApiFutures.addCallback() now requires a third argument (a non-direct executor). 

Ref: https://github.com/googleapis/api-common-java/blob/master/src/main/java/com/google/api/core/ApiFutures.java

google-cloud-examples/src/main/java/com/google/cloud/examples/pubsub/snippets/PublisherSnippets.java

@@ -36,6 +36,7 @@
 import com.google.pubsub.v1.ProjectTopicName;
 import com.google.pubsub.v1.PubsubMessage;
 import java.io.FileInputStream;
+import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 import org.threeten.bp.Duration;
 
@@ -62,7 +63,7 @@ public void onSuccess(String messageId) {
       public void onFailure(Throwable t) {
         System.out.println("failed to publish: " + t);
       }
-    });
+    }, Executors.newSingleThreadExecutor());
     return messageIdFuture;
   }