Remove SkEncodedInfo kUnknown_Color and kUnknown_Alpha from public API

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1918873002

Review URL: https://codereview.chromium.org/1918873002
7 files changed