ambient: fix photo attribution

Bug: b:169578241
Test: manually
Change-Id: Ia9ae5b9a71838c160ca81c73842c59b71db16270
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436583Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811403}

ash/ambient/backdrop/ambient_backend_controller_impl.cc

@@ -107,14 +107,11 @@ void BuildBackdropTopicDetails(
     const backdrop::ScreenUpdate::Topic& backdrop_topic,
     AmbientModeTopic& ambient_topic) {
   switch (backdrop_topic.topic_type()) {
-    case backdrop::TopicSource::CURATED:
-      ambient_topic.details = BuildCuratedTopicDetails(backdrop_topic);
-      break;
     case backdrop::TopicSource::PERSONAL_PHOTO:
       ambient_topic.details = BuildPersonalTopicDetails(backdrop_topic);
       break;
     default:
-      ambient_topic.details = std::string();
+      ambient_topic.details = BuildCuratedTopicDetails(backdrop_topic);
       break;
   }
 }