Expose the SkYUVAIndex version of the SkImage factories

TBR=bsalomon@google.com
Bug: skia:7903
Change-Id: Ib5a3bdba99da4cfb79115d1ff7b2c014272a5556
Reviewed-on: https://skia-review.googlesource.com/c/159520
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
6 files changed