[canvaskit] Expand SkAnimatedImage

With these things exposed, I think Flutter will not need
CanvasKit to expose the very complex SkCodec API.

Change-Id: Iace1b496d1dcb8842181466e860e8f212aba7b48
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253542
Reviewed-by: Leon Scroggins <scroggo@google.com>
6 files changed