Enable battery level reporting for bluetooth devices on linux

This is an experimental feature of BlueZ bluetooth stack. To enable the feature edit bluetooth.service and put the ExecStart to be /usr/libexec/bluetooth/bluetoothd --experimental.

The file bluetooth.service is present in /lib/systemd/system/ on debian. Otherwise, the file can be edited by issuing command sudo systemctl edit bluetooth.service.

After this change restart the bluetooth service by sudo systemctl daemon-reload && sudo systemctl restart bluetooth. The power status for the device connected via bluetooth should be now visible in power pane of GNOME Settings application.