Fix issues with downloaded images not getting loaded

lib/models/photo.dart

@@ -29,7 +29,9 @@ class Photo {
     photo.localId = asset.id;
     photo.title = asset.title;
     photo.pathName = pathEntity.name;
-    photo.createTimestamp = asset.createDateTime.microsecondsSinceEpoch;
+    photo.createTimestamp = asset.createDateTime.microsecondsSinceEpoch == 0
+        ? asset.modifiedDateTime.microsecondsSinceEpoch
+        : asset.createDateTime.microsecondsSinceEpoch;
     return photo;
   }
 

lib/photo_sync_manager.dart

@@ -57,7 +57,7 @@ class PhotoSyncManager {
         // "Recents" contain duplicate information on Android
         var assetList = await pathEntity.assetList;
         for (AssetEntity entity in assetList) {
-          if (entity.createDateTime.microsecondsSinceEpoch >
+          if (entity.modifiedDateTime.microsecondsSinceEpoch >
               lastDBUpdateTimestamp) {
             try {
               photos.add(await Photo.fromAsset(pathEntity, entity));