Use GNI group instead of hardcoding PNG codecs source files. (flutter#54781)

Fixes https://issues.skia.org/issues/362267329. The indirection
introduced by this PR insulates Flutter from source-file-list changes in
Skia. See the linked bug for more details.

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Author: anforowicz

skia/BUILD.gn

@@ -528,10 +528,8 @@ optional("png_decode") {
   ]
 
   deps = [ "//flutter/third_party/libpng" ]
-  sources = [
-    "$_skia_root/src/codec/SkIcoCodec.cpp",
-    "$_skia_root/src/codec/SkPngCodec.cpp",
-  ]
+  sources = [ "$_skia_root/src/codec/SkIcoCodec.cpp" ]
+  sources += skia_codec_png
 }
 
 optional("png_encode") {