The riff was Carlos's and you can hear him using it on the version of Footstompin/Wish I could Shimmy... that they did at the end of the 74 tour and also performed on Dick Cavett's TV show.

Bowie on Fame session with Lennon, speaking to Musician magazine in 1983:

MUSICIAN: How did the Fame session with John Lennon for the 1975 Young Americans LP come about?

BOWIE: After meeting in some New York club, we'd spent quite a few nights talking and getting to know each other before we'd even gotten into the studio. That period in my life is none too clear, a lot of it is really blurry, but we spent endless hours talking about fame, and what it's like not having a life of your own any more. How much you want to be known before you are, and then when you are, how much you want the reverse: "I don't want to do these interviews! I don't want to have these photographs taken!" We wondered how that slow change takes place, and why it isn't everything it should have been.
I guess it was inevitable that the subject matter of the song would be about the subject matter of those conversations. God, that session was fast. That was an evening's work! While John and Carlos Alomar were sketching out the guitar stuff in the studio, I was starting to work out the lyric in the control room. I was so excited about John, and he loved working with my band because they were playing old soul tracks and Stax things. John was so up, had so much energy; it must have been so exciting to always be around him.

Cool stuff.
And to apply what Bowie/Lennon/Alomar were saying about fame to the topic: I really think fame is a by-product that may or may not appear if we're doing our jobs properly - in other words it's kinda out of our control how the big media machine reacts. That most people have an Angel or a Blaine or a Copperfield at the tip of their tongues (rather than a Slydini or a Kaps or a Robert-Houdin) is better than having nothing at all and is probably an attempt to engage us in conversation with something - anything - as a contribution. Most lay people also have an inkling that there's a galaxy of highly accomplished magic heroes out there, and occasionally, I'll have someone come up and say something like: I saw Harry Blackstone Jr or Dai Vernon. It's not often, but it does happen.

When I was a lay person I wanted to see magic that amazed me, performed by somebody who was not creepy. I think being a sincere human being is underrated in magic.