[skottie] Ref-counted animators

Change-Id: I022c655c3f72551ccf6d35ba013fba6461e89a5c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229389
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
13 files changed