Release transport when endpoint connection property is updated to false.
This also checks if the transport is already created when receiving
endpoint properties update to prevent multiple transport creation.
The ConnectAudio method triggers Introspect and NewConnection calls from
hsphfpd to the backend, which will set the fd to use for the SCO data.
This imply to run the DBus loop while waiting for ConnectAudio reply to be
able to reply to those method calls.
MTU is hard-coded to 48. Using MTU from hsphfpd may prevent correct
sound operations.
Volume management is not yet implemented.
As hsphfpd is a prototype and its API subject to change, this backend is
disable by default.
This skeleton connects to hsphfpd daemon and list the managed devices.
