Add an option to mirror audio only

[Piripe]

• I have checked that a similar feature request does not already exist.

Is your feature request related to a problem? Please describe.
I would like to share the audio of my device without the display window.

Describe the solution you'd like
Allow the --no-display argument without a record or v4l2 to receive only the audio of the device.

Describe alternatives you've considered
Add another command argument (like --only-audio) to transmit only the audio of the device.

[rom1v]

I plan to add --no-video at some point in the future (so it would only transmit audio). It requires some work, because scrcpy has been designed for video since the beginning. That's why I haven't implemented it in 2.0.

[rom1v]

Please test #3978.