Hide filterquality enum entirely

Introduce private SkLegacyFQ to allow us to continue deserializing
old content with filter-qualities, but still remove the public enum.

Need to finish removing references in clients.

Change-Id: Iacfb602ec497260e53defc4b95e297ed1ef4e69c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/429476
Auto-Submit: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
11 files changed