Move SkImageGenerator::MakeFromEncoded to private

Last use by clients changed in
 - https://github.com/flutter/engine/commit/b0da68e7e024a20b1fdcc2490dca3d820f918999

Change-Id: I1c00c8b8cdc20223605bdb7cb3df5a8e1bb300e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/687644
Reviewed-by: Herb Derby <herb@google.com>
12 files changed