I do not recommend connecting a power source to the power leads of a cable going from the side port to a mobile device.
When the RC is connected to a mobile device using an OTG cable to the side port, the RC port is configured to GIVE power, not to receive it. By adding an external power source in such a configuration, you're forcing power in the opposite direction than designed.
Now if you connect the external power source to the power leads going to the mobile device and leave the power leads coming from the RC disconnected, that should be OK.
Google didn't provide a charge state option in their API for apps to use. There is a file that can be modified to change the state, but requires root. Some manufacturers have provided the function as they have root access in their adaptation to the Android kernel.
USB standards require a host (RC in this case) to provide at least 500ma available for a device, and the hardware most Androids use need to see power to enable data to be used. If you get the resistance just right, then current isn't quite sufficient to trigger the Android to charge, but not insufficient to enable use of data.