Reasoning is explained in the inline comment (basically, stable doesn't
care what the user specifies as the frame count when falling back to
skin resources).
This change also removes on to two layers of drawables, which should be
a win in heavy storyboards.
Usually this would be handled by `TextureAnimation`, but because we are
inheriting from `DrawableAnimation` here for reasons, we needed to
implement this ourselves. Follows the implementation in
`TextureAnimation`.