So, I have the 2016 Retina MacBook and encountered the same problem with the
Apple Multiport AV Adapter.
I've found three different solutions that definitely work:
One: use a
j5create charging bridge - it's a little box that has a USB-C plug, USB-C jack, and a USB-A jack. This lets you plug a single USB-A device into your MacBook while still connecting the USB-C power adapter.
Two: plug a
powered USB hub into the Apple Multiport adapter. Pretty much
any externally-powered USB 3.0 or 3.1 hub should work.
Three: get a
USB 3 Y cable, plug the main plug into the Apple Multiport adapter, and plug the other connector into a usb charger - basically, anything will work, a phone charger, a
portable power bank. (The link is to the one I'm using.)