move largest apis into private

Related to https://skia-review.googlesource.com/c/skia/+/91860

Bug: skia:
Change-Id: Ia8fd981b422bbab75541b078277d2e09e1fc9d41
Reviewed-on: https://skia-review.googlesource.com/91940
Reviewed-by: Brian Salomon <bsalomon@google.com>
23 files changed