It seems to be such a simple request... having a portable interface that works well with tracks on your phone. That order is apparently taller than we think. I have an iRig and it works perfectly with Apple devices (Garageband, Amplitube, etc), but it also won't satisfy your requirement. Amplitube works with backing tracks, but you have to import it into the app, and it's generally more schlepp than you'd expect.
This is probably a much better analogue option for use with headphones. And CHEAPER!
It has an Aux in that allows you to play the tracks from your phone.
http://www.musiciansfriend.com/amplifiers-effects/vox-amplug-2-classic-rock-guitar-headphone-amp