[skottie] Cleanup: relocate Animator base class out of SkSG

Decouple animator semantics from the scene graph.

No functional changes, just shuffling code around.

TBR=
Change-Id: I9a4592c424d2f8eb45b1b5f3e1f4b885cfaff9f6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/279421
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
17 files changed