Goal

This PR adds support for capturing and transmitting screen audio during screen sharing on Android. Previously, when users shared their screen, only the video was transmitted. This change enables capturing system audio (e.g., from media playback, other apps) and mixing it with microphone audio before transmission.

Please note that the screen audio is not send if user mutes their microphone since the local audio track is unpublished when muting.

Implementation

Screen Audio Capture:

• Added screen audio capture using AudioRecord with AudioPlaybackCaptureConfiguration (Android Q+)
• Implemented on-demand audio reading directly from AudioRecord using READ_BLOCKING mode
• Screen audio is read synchronously when requested by the WebRTC audio callback

Audio Mixing:

• Created addAndConvertBuffers() utility function to mix microphone and screen audio buffers
• Integrated mixing logic in StreamPeerConnectionFactory.setAudioBufferCallback():
  • Audio filters are applied to microphone audio first
  • Screen audio is then mixed with filtered microphone audio
  • When microphone is muted, screen audio is muted as well
  • When microphone is enabled, both are mixed together

Audio Track Management:

• Added includeAudio parameter to Call.startScreenSharing() (defaults to false)
• Added audioEnabled StateFlow to ScreenShareManager to track screen share audio state separately from video

Key Changes:

• MediaManager.ScreenShareManager: Added screen audio capture infrastructure and audioEnabled StateFlow
• StreamPeerConnectionFactory: Added audio mixing logic with proper filter application order
• Demo app: Updated to pass includeAudio=true when starting screen sharing

🎨 UI Changes

None

Testing

On the demo app :

1. Start screen sharing with includeAudio=true and verify system audio is captured and transmitted
2. Mute microphone while screen sharing with audio enabled - verify screen audio also stops on the other side
3. Unmute microphone while screen sharing - verify both microphone and screen audio are mixed and transmitted
4. Join call with microphone muted, then start screen sharing with audio - verify audio track is not heard on the other side but once mic is unmuted it is transmitted