Initialize a duplex stream properly when using aggregate device

[ChunMinChang]

A problem is found when investigating BMO 1631814.

The properties of a duplex stream are initialized with different devices when we use an internal aggregate device. The AudioUnit with out_dev_info but current_latency_frames and output_source_listener will be set via output_device. The output_device and out_dev_info are different values when we use an internal aggregate device for the duplex stream.

[ChunMinChang]

I've tested this today. I replace all the devices that bind to the property-changed callbacks in install_device_changed_callback by the aggregate-device that is being used as the duplex device within the cubeb stream. The test I used is in PR #94.

However, after testing, I wonder if it's intended to use the output or input device directly rather than the aggregate device when registering those listeners.

@padenot: In the duplex stream case, do those listeners register with the output or input devices, instead of the aggregate device, on purpose?

The aggregate-device we use for the duplex stream has only one input and only one output. When the output or input is unplugged, we should fire the device-changed event to reinitialized the stream. However, this seems the same as what the current code does. I am not sure if install_device_changed_callback needs to make a change. What do you think?