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

DsHidMini/include/DsHidMini/Ds3Types.h

Line 216 in 98a1ecd

UCHAR GEN_GamePadBatteryStrength; // Usage 0x00060020: Battery Strength, Value = 0 to 255

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