This updates FadeInImage to use the new Image.frameBuilder
API (added in #33369), to greatly simplify the implementation
of FadeInImage.

This also removes the FadeInImage.placeholderSemanticLabel property.
This property was added in #28799 for the sake of completeness (the bug
it fixed was the lack of any semantic label support in FadeInImage), but a
placeholder is a transient visual artifact, not something that affects the
underlying semantic meaning of the image.