[skottie] Switch to SkJSON

Convert to SkJSON APIs and remove RapidJSON dependency.

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Change-Id: I5b4fd78821a443326e3a5b370748d840b80ef279
Reviewed-on: https://skia-review.googlesource.com/134612
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
11 files changed