Unify XCAsset handling logic

Summary: @​public

This diff unifies the logic for detecting when images refer to XCAsset files into a single function (RCTXCAssetNameForURL) and uses it for both +[RCTConvert UIImage:] and RCTImageLoader.

I've also tightened the definition of XCAssets so that it only applies to images inside .car files, not any image inside the main bundle. This avoids using the +[UIImage imageNamed:] when not strictly necessary, which is desirable since that method is not thread-safe, and has undocumented caching behavior that is difficult to reason about.

Reviewed By: @javache

Differential Revision: D2526400

fb-gh-sync-id: 7199c2a44f1d55ff236d2c38a0a9368739b993d5

Libraries/Image/RCTXCAssetImageLoader.h

@@ -0,0 +1,14 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import "RCTImageLoader.h"
+
+@interface RCTXCAssetImageLoader : NSObject <RCTImageURLLoader>
+
+@end