USB To 3.5mm Jack mircophone
I assume the USB connection isn't the main point? You could look into a TRRS splitter to fit your 3,5mm plug into the headset jack on your phone. Here's one example, just to give you an idea of what I mean.

This way you can use a normal recording app to capture analog audio from your soundboard. Just be careful to check the levels since the jack isn't a line in jack, but a mic in jack.