Report battery level in XI mode

[nefarius]

Apparently there's a battery byte in the xinputhid report we can populate so other established apps can be used to read it in XI mode, see https://github.com/nefarius/DsHidMini/blob/98a1ecd3ec89cf5c01348f6d451f81502adf5cab/include/DsHidMini/Ds3Types.h#L216

Potential test app https://github.com/matracey/XInputBatteryMeter