livekit_client: Always use output device's reported channels num (#51276)

Fixes a panic in livekit's process_reverse_stream for non-matching
channel counts, e.g., upmixing 2->4, or downmixing 2->1.

Release Notes:

- Fixed a panic in livekit when joining a channel using legacy audio
using a device with less or more than 2 channels.

crates/livekit_client/src/livekit_client/playback.rs

@@ -258,7 +258,7 @@ impl AudioStack {
         apm: Arc<Mutex<apm::AudioProcessingModule>>,
         mixer: Arc<Mutex<audio_mixer::AudioMixer>>,
         sample_rate: u32,
-        num_channels: u32,
+        _num_channels: u32,
         output_audio_device: Option<DeviceId>,
     ) -> Result<()> {
         // Prevent App Nap from throttling audio playback on macOS.
@@ -270,6 +270,7 @@ impl AudioStack {
             let mut device_change_listener = DeviceChangeListener::new(false)?;
             let (output_device, output_config) =
                 crate::default_device(false, output_audio_device.as_ref())?;
+            info!("Output config: {output_config:?}");
             let (end_on_drop_tx, end_on_drop_rx) = std::sync::mpsc::channel::<()>();
             let mixer = mixer.clone();
             let apm = apm.clone();
@@ -300,7 +301,12 @@ impl AudioStack {
                                     let sampled = resampler.remix_and_resample(
                                         mixed,
                                         sample_rate / 100,
-                                        num_channels,
+                                        // We need to assume output number of channels as otherwise we will
+                                        // crash in process_reverse_stream otherwise as livekit's audio resampler
+                                        // does not seem to support non-matching channel counts.
+                                        // NOTE: you can verify this by debug printing buf.len() after this stage.
+                                        // For 2->4 channel upmix, we should see buf.len=1920, buf we get only 960.
+                                        output_config.channels() as u32,
                                         sample_rate,
                                         output_config.channels() as u32,
                                         output_config.sample_rate(),