Instead of a flat animator space, introduce animator groups.
This allows us to encapsulate layer animators and only dispatch ticks
when their owning layer is active.
TBR=
Change-Id: I1fc8a55abf68a712b71969bb1a11275dbe54c236
Reviewed-on: https://skia-review.googlesource.com/101201
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>