[skottie] Expose composition markers to embedders

Add a MarkerObserver interface (to replace the current
AnnotationObserver), and update CustomPropertyManager to intercept both
properties and markers.

TBR=
Change-Id: If79de419066916bc596316f0a551c75564069239
Reviewed-on: https://skia-review.googlesource.com/c/173766
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
8 files changed