Load local assets synchronously to prevent image flicker

Summary:
This uses `[UIImage imageNamed:]` to load local assets that are bundled using `require('../image/path.png')` and makes sure it is done synchronously on the main queue to prevent images from flickering. This improves user experience a lot when using large local images and prevents icon flickers to match the behaviour of most native apps.

This adds to methods to the ImageLoader protocol, one to tell if the image loader must be executed on the url cache queue and one to tell if the result of the image loader should be cached. I then use these to make the LocalImageLoader bypass the url cache queue and avoid caching images twice.

Note that this doesn't affect debug builds since images are loaded from the packager.

I'm not sure if we want to still support async loading of local images as I'm not sure how much of a perf difference this will make. Maybe someone at fb can benchmark this see how it affects your apps but there wasn't a noticeable one in mine. Also I only enabled this for loading png and jpg im
Closes facebook/react-native#8102

Reviewed By: bnham

Differential Revision: D3433647

Pulled By: javache

fbshipit-source-id: 37bd6aff20c0465c163db3cdbcaeaedff55f7b1f

UIExplorer/js/ImageExample.js

@@ -589,6 +589,20 @@ exports.examples = [
     },
     platform: 'android',
   },
+  {
+    title: 'Legacy local image',
+    description:
+      'Images shipped with the native bundle, but not managed ' +
+      'by the JS packager',
+    render: function() {
+      return (
+        <Image
+          source={require('image!hawk')}
+          style={styles.base}
+        />
+      );
+    },
+  },
 ];
 
 var fullImage = {uri: 'http://facebook.github.io/react/img/logo_og.png'};