Minimum setup for a stereo location recording

[if_then_else]

Correct, I don't know of any Android phones with stereo inputs.

Is there any software that can use the USB connector on an Android phone for an external A/D converter ?

Yes, the 'Field Recorder' app that I've linked above.
I've got it - but so far I haven't used it for live concerts.

There's a compatibility list of USB mics and audio interfaces on the website (only in German though).
http://pfitzingervoicedesign.com/field_recorder/usb_audio.html

[2manyrocks]

Look at krvaudio.com and be aware that they caution it's very device specific as to whether it will work with a particular device.