This problem is solved pretty easily:

1> Buy a shit Nokia, use the 2 pin charger. TBH if it goes flat it won't charge off USB anyway. Then use a microSD for transferring music/photos.

2> Buy an Android handset with USB OTG. Transfer files on and off via a USB stick that you control. Charge it with a USB cable with the data pins shorted (you can buy these off Amazon).

Both of these ignore the main attack vector that I'd be concerned about which is over-the-air (via GSM/packet data) and a fuck load more scary than plugging your USB thing into a dirty USB hole and getting some USB STD.