Make SkAnimatedImage::update return sentinel when stopped

Bug: b/63908092
Change-Id: I585f3d3efde1db4d5be36b11f19fe6e88f131608
Reviewed-on: https://skia-review.googlesource.com/98062
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
3 files changed