[skottie] Cleanup KeyframeAnimatorBuilder

Instead of plumbing the target value through bindImpl as an opaque
void*, store explicitly in builders.

More typesafe/elegant/flexible/etc.

TBR=
Change-Id: Ie28787072a6be3b0bfcd528b68431f9fb3fa3a71
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/291576
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
9 files changed