We allow vendor-specific icon installing image but have defined private
animation_fps that can't be overridden. This CL changes the image
generator to optionally embed FPS (otherwise use the default value of
20) into the generated image.

For wear devices, they are using individual images instead of the
interlaced one. Change the animation_fps from private to protected so
that it can be customized.

Bug: 26009230
Change-Id: I9fbf64ec717029d4c54f72316f6cb079e8dbfb5e