Siri support for per-app volume feature

[rokartur]

Due to how the Per-app Volume feature works in BetterAudio, Siri detects that its audio is being intercepted by ProcessTap and switches to text mode (for security/privacy reasons). It is impossible to control the volume for Siri only. If the app worked with the HAL driver and a virtual device, this would be possible (version 26.4). But then the user experience would be worse, because they would have to figure out how and when to set the virtual device as the default output for other apps, and my app wants to avoid that. I want my app to be easy to use and accessible to everyone.