make paint flatten unflatten private

SkPaint flatten and unflatten rely
on interfaces that are not public,
SkReadBuffer and SkWriteBuffer.

R=reed@google.com
Bug: skia:6172
Change-Id: I487af9f8931c78daf763c51d9e0d3ff8ff440b22
Reviewed-on: https://skia-review.googlesource.com/112561
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Cary Clark <caryclark@skia.org>
7 files changed