Filter devices with writable Core Audio volume control

[rustydomino]

Avoid creating SoundBridge proxies for output devices that already expose
writable Core Audio volume control.

This also improves volume capability detection by:

• determining the actual number of output channels from the device's stream
  configuration
• checking the main volume element and every reported output channel
• requiring kAudioDevicePropertyVolumeScalar to be settable, rather than
  treating the mere existence of the property as usable volume control

Devices without writable Core Audio volume control continue to be accepted.

Tested with:

• fixed-volume DisplayPort monitor: accepted
• USB output with writable Core Audio volume: skipped
• Host release build- Improve audio volume capability detection
• Filter devices with writable volume control