Fix bug in SkGifCodec / Switch SkImageDec tests to use Codec

cherry of 7f7ec206de39fde8dc490e9feb0f65322af1b989 in master

Original commit message below:

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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
NOTREECHECKS=true
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1757333002
12 files changed