[skottie/cleanup] Refactor parser state handling

  * introduce AnimationBuilder to hold mostly immutable (modulo caching)
    state
  * split the scoped animator state into AnimatorScope

This will facilitate splitting the monolithic Skottie.cpp in follow-up
CLs.

Refactoring only, no functional changes.

TBR=
Change-Id: I0a8295e60be4559586fc4a9fea3dee4a7f5714d4
Reviewed-on: https://skia-review.googlesource.com/148390
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
5 files changed