Don't synchronize MMS groups to sibling devices

Fixes #6934
// FREEBIE

src/org/thoughtcrime/securesms/jobs/MultiDeviceGroupUpdateJob.java

@@ -64,6 +64,7 @@ public class MultiDeviceGroupUpdateJob extends MasterSecretJob implements Inject
       reader = DatabaseFactory.getGroupDatabase(context).getGroups();
 
       while ((record = reader.getNext()) != null) {
+        if (!record.isMms()) {
           List<String> members = new LinkedList<>();
 
           for (Address member : record.getMembers()) {
@@ -74,6 +75,7 @@ public class MultiDeviceGroupUpdateJob extends MasterSecretJob implements Inject
                                     members, getAvatar(record.getAvatar()),
                                     record.isActive()));
         }
+      }
 
       out.close();