Add the hasSource method to the AudioMixer interface.

PiperOrigin-RevId: 551804976

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

@@ -106,6 +106,11 @@ public interface AudioMixer {
    */
   int addSource(AudioFormat sourceFormat, long startTimeUs) throws UnhandledAudioFormatException;
 
+  /**
+   * Returns whether there is an {@link #addSource added source} with the given {@code sourceId}.
+   */
+  boolean hasSource(int sourceId);
+
   /**
    * Sets the volume applied to future samples queued from the given source.
    *

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

@@ -151,6 +151,7 @@ import java.nio.ByteOrder;
     return sourceId;
   }
 
+  @Override
   public boolean hasSource(int sourceId) {
     checkStateIsConfigured();
     return containsKey(sources, sourceId);