Add --audio-dup

Add an option to duplicate audio on the device, compatible with the new
audio playback capture (--audio-source=playback).

Fixes #3875 <https://github.com/Genymobile/scrcpy/issues/3875>
Fixes #4380 <https://github.com/Genymobile/scrcpy/issues/4380>
PR #5102 <https://github.com/Genymobile/scrcpy/pull/5102>

Co-authored-by: Simon Chan <1330321+yume-chan@users.noreply.github.com>