Use separate object for multi recipient response

`needsSync` was being sent back from the server in the JSON response
which is an unnecessary and constantly false field in multi-recipient
message sending endpoint as it's always sealed sender.

service/src/main/java/org/whispersystems/textsecuregcm/controllers/MessageController.java

@@ -76,6 +76,7 @@ import org.whispersystems.textsecuregcm.entities.MultiRecipientMessage.Recipient
 import org.whispersystems.textsecuregcm.entities.OutgoingMessageEntity;
 import org.whispersystems.textsecuregcm.entities.OutgoingMessageEntityList;
 import org.whispersystems.textsecuregcm.entities.SendMessageResponse;
+import org.whispersystems.textsecuregcm.entities.SendMultiRecipientMessageResponse;
 import org.whispersystems.textsecuregcm.entities.StaleDevices;
 import org.whispersystems.textsecuregcm.limits.RateLimitChallengeException;
 import org.whispersystems.textsecuregcm.limits.RateLimitChallengeManager;
@@ -417,7 +418,7 @@ public class MessageController {
         uuids404.add(destinationAccount.getUuid());
       }
     }
-    return Response.ok(new SendMessageResponse(uuids404)).build();
+    return Response.ok(new SendMultiRecipientMessageResponse(uuids404)).build();
   }
 
   private void checkAccessKeys(CombinedUnidentifiedSenderAccessKeys accessKeys, Map<UUID, Account> uuidToAccountMap) {

service/src/main/java/org/whispersystems/textsecuregcm/entities/SendMessageResponse.java

@@ -6,24 +6,15 @@
 package org.whispersystems.textsecuregcm.entities;
 
 import com.fasterxml.jackson.annotation.JsonProperty;
-import java.util.List;
-import java.util.UUID;
 
 public class SendMessageResponse {
 
   @JsonProperty
   private boolean needsSync;
 
-  @JsonProperty
-  private List<UUID> uuids404;
-
   public SendMessageResponse() {}
 
   public SendMessageResponse(boolean needsSync) {
     this.needsSync = needsSync;
   }
-
-  public SendMessageResponse(List<UUID> uuids404) {
-    this.uuids404 = uuids404;
-  }
 }

service/src/main/java/org/whispersystems/textsecuregcm/entities/SendMultiRecipientMessageResponse.java

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.entities;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import java.util.List;
+import java.util.UUID;
+
+public class SendMultiRecipientMessageResponse {
+  @JsonProperty
+  private List<UUID> uuids404;
+
+  public SendMultiRecipientMessageResponse() {
+  }
+
+  public SendMultiRecipientMessageResponse(final List<UUID> uuids404) {
+    this.uuids404 = uuids404;
+  }
+}