I know Apple employees are passionate Apple fans. I do have an old iPhone that I can use. Should I just bring my old iPhone with me to the interview?
In general, I agree with Snow. You're applying for a position at Apple though, questions about your favourite brand may show up. Answer them truthfully. You don't want to say, "I'm currently using an iPhone", refering to your old phone, when in reality you're using an Android on a daily basis. They'll notice on your first day anyway. Try to spin it around in a good way. As an example, I heard someone mention this in a conversation just yesterday.
I wish I had gone for another iPhone instead of this Android. It seemed good on paper, but I really miss features X and Y from iPhones.
If it is relevant for you, you could use this phrase, or another one that is positive and true.