Fix bug in SkGifCodec / Switch SkImageDec tests to use Codec

SkImageDecoder is still used throughout tests, tools, gms etc.
Deleting it from tests is an easy first step.

Bonus is that we add tests of SkCodec.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1733863003

Review URL: https://codereview.chromium.org/1733863003
12 files changed