The answer is that DJI Go currently does not allow you to turn off charging for Android devices. If you want longer RC battery life, go back to the iPad (making sure the setting to charge the tablet is OFF for the iOS device).
Since there was a recent firmware update, you can check if they changed that, but I haven't heard that they have. It's really kind of a dumb bug.
It's not DJI that make this choice it's because they follow the USB standard which doesn't allow this functionality whereas Apple use their proprietary lightning connection that does. The modified cables that are available that do disable power but allow data are not USB standard, the charging rate is a minimal 500mA which is barely a trickle and in normal use it's not an issue with the controller easily able to last through three batteries. I used the tablet with the controller for around a year with no issues and the battery percentage still drops very quickly even without a device attached, on its own it has far less batterylife than when I used to use it with the tablet.
There was no firmware update performed on the controller before this behaviour started.