The key word here is "partly" Swift. I would not put it past Apple to use an intermediary framework that hosts a Swift runtime and calls into native Android APIs. That keeps the non-UI logic in one codebase.
That's more or less what they do with iTunes, isn't it? I recall there being an incomplete library packaged with iTunes once upon a time (with stuff like a stub implementation of Grand Central Dispatch that was neither grand nor dispatching).