Replace skia_encode_png_srcs with more granular gni lists.

[anforowicz]

skia_encode_png_base + skia_encode_libpng_srcs is the same as the legacy skia_encode_png_srcs. The latter will be removed from Skia soon.

This is motivated by https://crbug.com/381900683 and the desire to enable using skia_encode_png_base with either
skia_encode_libpng_srcs or (currently experimental) skia_encode_rust_png_srcs.

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[anforowicz]

Ooops... I am guessing that https://skia-review.googlesource.com/c/skia/+/930419 from Skia hasn't yet been rolled into Flutter. I think this means we just need to wait.

/cc @kjlubick