Get battery status using Apple HFP AT commands on RFCOMM socket and send them to BlueZ experimental Battery Provider API. Applies only to native backend since oFono and hsphfpd manage RFCOMM socket themselves.
Resolves #382 (closed) .
Tested with Sony WH-CH400, Pop!_OS 20.04 (running BlueZ 5.56 from Fedora package manually with
TODO: also report status as a node property.