Fix preview of previously offline images

Signed-off-by: ZetaTom <70907959+ZetaTom@users.noreply.github.com>

app/src/main/java/com/owncloud/android/datamodel/ThumbnailsCacheManager.java

@@ -204,7 +204,10 @@ public final class ThumbnailsCacheManager {
         thumbnail = BitmapUtils.rotateImage(thumbnail,path);
 
         // Add thumbnail to cache
-        addBitmapToCache(imageKey, thumbnail);
+        // do not overwrite any pre-existing image
+        if (!mThumbnailCache.containsKey(imageKey)) {
+            addBitmapToCache(imageKey, thumbnail);
+        }
 
         return thumbnail;
     }