[skottie] Cleanup: track animator scoping in AnimationBuilder

Instead of passing an explicit scope all over, keep track of the current
scope in AnimationBuilder.

Removes a bunch or redundant plumbing.

TBR=
Change-Id: I9e587f4ae7a1d12f86d13f30144816492a4ce147
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229762
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
29 files changed