Initial Lottie loader impl (Skotty)

Coarse workflow:

* Construction

  1) build a Json tree
  2) collect asset IDs (for preComp/image layer resolution)
  3) "attach" pass
     - traverse the Json tree
     - build an SkSG dom, one fragment at a time
     - attach "animator" objects to the dom, for each animated prop
  4) done, we can throw away the Json tree

* For each animation tick

  1) iterate over active animators and poke their respective dom nodes/attributes
  2) revalidate the SkSG dom
  3) draw the SkSG dom

Note: post construction, things are super-simple - we just poke SkSG DOM attributes
with interpolated values, and everything else is handled by SkSG (invalidation,
revalidation, render).

Change-Id: I96a02be7eb4fb4cb3831f59bf2b3908ea190c0dd
Reviewed-on: https://skia-review.googlesource.com/89420
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>

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