iconitem: Adjust fade animation to not flicker as much

As it turns out, mix(old texture, new texture, animation value) does actually result in a different visual from simply blending the two textures on top of each other, because the old texture first gets blended with the underlying color. This results in some of the underlying color bleeding through, which can cause flickering if the two textures are almost the same. To avoid this, change the animation slightly so that rather than crossfading, we first fade in the new texture then fade out the old texture. This doesn't cause a whole lot of visual difference for two completely different textures, but removes the flickering when they are very similar.

BUG: 463061 BUG: 463685 FIXED-IN: 5.102