Remove SkMath.h from the public API

This had been waiting on:
 - http://ag/21022503
 - http://go/cl-c/4174994

Change-Id: I4aa3a750a52a05a0dcd9ac8336f9239bfaf6876a
Bug: skia:13983
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/631239
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed