Simplify SkAnimatedImage

Bug: b/63908092

Rather than keeping track of the time and whether the animation is
running, leave that up to the client. Offer a single method to decode
the next frame, allowing the client to stay one frame ahead.

Change-Id: I546013e32e3a0874181b0dce1349bbec07aaadd4
Reviewed-on: https://skia-review.googlesource.com/101544
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
4 files changed