[skottie] Use a documenting alias for Animator::seek() return type

Change-Id: I46aeddf1e0739cfac9967e29192d542d6222c6ee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/280343
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>