bluez5: cleanup hfp/hsp backend handling + config file option

It seems not uncommon that people have not properly configured ofono
running, which results to loss of HFP/HSP functionality. It's less
surprising if the backend selection is fixed in the configuration file,
and (by default) does not depend on running services.

Add a configuration file option for selecting HFP/HSP backend, and set
the default value to the native backend. Emit warnings if conflicting
backend services are detected to be running.

Also cleanup hfp/hsp backend handling a bit, now that it's mostly
abstracted behind an interface.