Make SkColorSpace a public API

I've trimmed uniqueID for now, and added some comments.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1996973002

Review-Url: https://codereview.chromium.org/1996973002
6 files changed