Rename SkAutoFree -> UniqueVoidPtr and move to private

Change-Id: I0a4385439ddd13dc6e6667c1a61d425be93a4c28
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/633576
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Herb Derby <herb@google.com>
6 files changed