Use SkSpan in read and write buffers

Change-Id: I220d77f203f6314c545b501b5f72be74f1c683ad
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1004476
Reviewed-by: Daniel Dilan <danieldilan@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Mike Reed <mike@reedtribe.org>