bluetooth: Remove device_is_audio_ready()

bluetooth: Remove device_is_audio_ready()

The function was used to check whether the basic properties of the
Bluetooth device have been received. This can be simplified by just
checking d->device_info_valid, since the state of the audio interface
is only relevant inside pa_bluetooth_device_any_audio_connected(), which
is used to trigger the discovery callback.

While checking device_info_valid, special care must be taken with all
three possible values: when set to -1, it means some error was triggered
while getting the device properties. Therefore, these devices can also
be ignored outside bluetooth-util.

Besides that, the patch slightly modifies the behavior of the internal
API affecting pa_bluetooth_discovery_get_by_address() and
pa_bluetooth_discovery_get_by_path(), since they will return the device
no matter the state of the audio interface. This however makes sense and
should have no influence in the current codebase given that the modules
make use of devices only after the discovery hook has been triggered.