SkAnimatedImage: Use fSampleSize

Bug: b/163595585

This will allow using less memory when decoding an animated GIF by
sampling at decode time. This is tested by the animated_image GMs.

Change-Id: I748b2180827623e4ca1fc0fd4d6dd02733b3b5f2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/333226
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
2 files changed