[skottie] pass sid if present to PropertyObserver

We update the context passed to the property observer for fill strokes.
sid's are always found for fill strokes under `c`.
This CL also marks places where we will need to pull the `sid` of the node.

Change-Id: Ice65c1d8f3e5ee6c4af617812d9e7fba88c0bbe9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/670116
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
6 files changed