Reland: [skottie] Initial property setters

Introduce a PropertyObserver to receive property notifications for layer
and shape nodes.

Properties are communicated using strongly-typed "handles", which act
as impedance adapters between the AE/BM model and the internal Skottie
model.

Reviewed-by: Mike Reed <reed@google.com>
Change-Id: Id155076faa8595f6b4d81672559f01c2e0c7455a
TBR=
Reviewed-on: https://skia-review.googlesource.com/156626
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
11 files changed