If you're going to buy a flagship Android device when it first comes out (and pay $200 upfront) and try and sell it... why not just pay the $200 upfront you would anyways and buy the iPhone that you want to begin with?
That's an okay plan and all, but ultimately I feel you'll lose more money than it's worth. As a long time android user I can tell you from what I've seen that Android phones only retain high values right when they come out (for like a month or two) or when they are supply constrained. Otherwise, their value drops off significantly after a couple months. You'd be better off buying a used android phone that works on AT&T that came out in the last 6-9 months and using that, then selling it for little loss.
For example, years ago when my family switched to T-Mobile I picked up a Galaxy S3 with the intentions of selling it, as I had a Galaxy Nexus. This phone was brand new, and had been out for around 4 months and only fetched me $430 whereas the original price was $650 and on contract was $99. If you did something similar, and intended to buy the 6S, you'd still need to fork over around $400 about 3 months from now.