move a bunch of helpers from SkImageInfo.h into priv

Bug: skia:
Change-Id: I8c91cfdb89e4f22448d1201d391556fe43d86dca
Reviewed-on: https://skia-review.googlesource.com/105289
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Cary Clark <caryclark@google.com>
17 files changed