Fix RCTImageLoader's priority

Summary:
There is a conflict between RCTImageLoader and RCTAssetsLibraryRequestHandler on handling images, which was caused by making RCTPhotoLibraryImageLoader able to handle assets library requests as well. This gives more priority to RCTImageLoader instead.
Fixes #9031.

Reviewed By: mmmulani

Differential Revision: D3627451

fbshipit-source-id: 7ffd2c66f43ce1479c9a117768fb2d29f9d0dc08

Libraries/Image/RCTImageLoader.m

@@ -85,6 +85,11 @@ - (void)dealloc
   [[NSNotificationCenter defaultCenter] removeObserver:_decodedImageCache];
 }
 
+- (float)handlerPriority
+{
+  return 1;
+}
+
 - (id<RCTImageURLLoader>)imageURLLoaderForURL:(NSURL *)URL
 {
   if (!_maxConcurrentLoadingTasks) {