ImageMetaPropsPayload (for notifications) was supposed to contain
exactly the same fields as ImageMetaProps itself; and the SCHEMA member
was those same keys in a trivially different shape. And we were having
trouble remembering to keep these two objects in sync before. So this
commit eliminates the repetition and just generates
ImageMetaPropsPayload's `fields` and `SCHEMA` from
ImageMetaProps.fields.
Change-Id: I7dc38f7f5a62098fb0c366736f27e23bd7b7ee86
bce0854959