Don't constrain audio for playout in Transformer

Transformer's input shouldn't be constrained to the number of playable audio channels on the current device because the media may be edited (to mix channels for example) or encoded for playback on another device (a server for example).

PiperOrigin-RevId: 625604243

RELEASENOTES.md

@@ -13,6 +13,8 @@
 *   Transformer:
     *   Work around a decoder bug where the number of audio channels was capped
         at stereo when handling PCM input.
+    *   When selecting tracks in `ExoPlayerAssetLoader`, ignore audio channel
+        count constraints as they only apply for playback.
 *   Track Selection:
 *   Extractors:
 *   Audio:

libraries/transformer/src/main/java/androidx/media3/transformer/ExoPlayerAssetLoader.java

@@ -160,6 +160,7 @@ public final class ExoPlayerAssetLoader implements AssetLoader {
     trackSelector.setParameters(
         new DefaultTrackSelector.Parameters.Builder(context)
             .setForceHighestSupportedBitrate(true)
+            .setConstrainAudioChannelCountToDeviceCapabilities(false)
             .build());
     // Arbitrarily decrease buffers for playback so that samples start being sent earlier to the
     // exporters (rebuffers are less problematic for the export use case).