[skottie] Relocate text shaping utils to separate compilation unit

Introduce AE-specific text shaping abstractions, and move the existing
implementation to a separate CU.

Change-Id: I8533ed8d9c59af6d82b3f1ca66a6e1d50a3c33c5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/203706
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
5 files changed