Consolidate some helpers under skottie_utils, and update all related
tools to support animated images.
TBR=
Change-Id: If08e97143a11d9a414f3230a49ab4284c508b9d0
Reviewed-on: https://skia-review.googlesource.com/c/169342
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
