Fix ViewConfig validation error after merging defaultSrc into default…

…Source for RCTImageView

Summary:
The `./resolveAssetSource` processor was added to the `defaultSource` prop in the `RCTImageView` static view config in D65819218.
To match this in native view config, we're adding the `customType = "ImageSource"` hint to the view config infra, so it knows to assign correct processor to the native view config in runtime.

Changelog: [Internal]

Differential Revision: D66228921

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/image/ReactImageManager.kt

@@ -124,7 +124,7 @@ public constructor(
     }
   }
 
-  @ReactProp(name = "defaultSource")
+  @ReactProp(name = "defaultSource", customType = "ImageSource")
   public fun setDefaultSource(view: ReactImageView, source: String?) {
     view.setDefaultSource(source)
   }