bluetooth: Create pa_bluetooth_transport for BlueZ 5 support
Create the pa_bluetooth_transport structure to store information about
the bluetooth transport and utility functions to manipulate this
structure. The acquire() and release() operations are function pointers
in the pa_bluetooth_transport structure to make possible for different
transport backends to provide different implementations of these
operations. Thre is also a userdata field for the transport backend
provide data for the acquire/release functions.
This commit also creates a new function
pa_bluetooth_device_any_transport_connected() to check if there is any
audio connection between the host and a remote device.