move SkMipmapBuilder to private

Change-Id: I77f57b175984a5e373fae951a270a883e7e4c5d3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/339216
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
7 files changed