Yeah, that's for Bonnies album. She gave an interview a couple of weeks ago where she listed people she had worked with on it and Katys name was amongst them.
"As for her own music, McKee explained that she’s working with “all kinds of people” on her new material: “Greg Kurstin, Doctor Luke, Katy Perry, I’ll be writing with -- and several people from the Teddybears.” She alluded to some additional “well-known” collaborations."
So we could very well be hearing a Bonnie feat. Katy song very soon! It's cute that Katy's writing on Bonnies album now rather than it always being the other way around.