Make ios bitmap chroma sampling mode 420 by default.

Summary: Before adding a none case, returning 420 from bitmap to ensure images are saved as 420 when `propagateSamplingModeFromSource` is on.

Reviewed By: lambdapioneer

Differential Revision: D13319314

fbshipit-source-id: 2be68796852b8a6e82eb2029a0dad22cb0d2a4c7

ios/SpectrumKit/SpectrumKit/Image/FSPImageSpecification.mm

@@ -128,7 +128,7 @@ + (instancetype)imageSpecificationFromImage:(UIImage *)image
                              format:FSPImageFormat.bitmap
                  pixelSpecification:[FSPImagePixelSpecification imagePixelSpecificationFromImage:image]
                         orientation:FSPImageOrientationFromImage(image)
-                 chromaSamplingMode:FSPImageChromaSamplingMode444
+                 chromaSamplingMode:FSPImageChromaSamplingMode420
                            metadata:[FSPImageMetadata imageMetadataFromImage:image]];
 }