Don't worry too much about what platform you initially develop your app for -- if you can validate your concept then it shouldn't be hard to find assistance porting it to other platforms.
Too many people get tied up on "what platform should I develop for?" when they could be spending that time getting an MVP up on _something_, validating it and getting the ball moving.
Too many people get tied up on "what platform should I develop for?" when they could be spending that time getting an MVP up on _something_, validating it and getting the ball moving.