Use reference-counting for listeners from MediaSourceEventDispatcher

Previously calling removeListener would remove all instances of that
listener. Now it only removes a single instance.

This probably should have been part of introducing the Multiset:
https://github.com/google/ExoPlayer/commit/2bd4d61b9b8c838fb7dc81e7d6f513c4a4e00c76

PiperOrigin-RevId: 301191940

library/core/src/main/java/com/google/android/exoplayer2/util/MediaSourceEventDispatcher.java

@@ -117,6 +117,10 @@ public class MediaSourceEventDispatcher {
   /**
    * Removes a listener from the event dispatcher.
    *
+   * <p>If there are duplicate registrations of {@code {eventListener, listenerClass}} this will
+   * only remove one (so events dispatched to {@code listenerClass} will still be passed to {@code
+   * eventListener}).
+   *
    * @param eventListener The listener to be removed.
    * @param listenerClass The listener type passed to {@link #addEventListener(Handler, Object,
    *     Class)}.
@@ -126,6 +130,7 @@ public class MediaSourceEventDispatcher {
       if (listenerInfo.listener == eventListener
           && listenerInfo.listenerClass.equals(listenerClass)) {
         listenerInfos.remove(listenerInfo);
+        return;
       }
     }
   }

library/core/src/test/java/com/google/android/exoplayer2/util/MediaSourceEventDispatcherTest.java

@@ -20,6 +20,7 @@ import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
 
 import android.os.Handler;
 import android.os.Looper;
@@ -162,6 +163,26 @@ public class MediaSourceEventDispatcherTest {
     verify(mediaAndDrmEventListener).onMediaPeriodCreated(WINDOW_INDEX, MEDIA_PERIOD_ID);
   }
 
+  @Test
+  public void listenersAreCountedBasedOnListenerAndType() {
+    // Add the listener twice and remove it once.
+    eventDispatcher.addEventListener(
+        Util.createHandler(), mediaSourceEventListener, MediaSourceEventListener.class);
+    eventDispatcher.addEventListener(
+        Util.createHandler(), mediaSourceEventListener, MediaSourceEventListener.class);
+    eventDispatcher.removeEventListener(mediaSourceEventListener, MediaSourceEventListener.class);
+
+    eventDispatcher.dispatch(
+        MediaSourceEventListener::onMediaPeriodCreated, MediaSourceEventListener.class);
+
+    verify(mediaSourceEventListener).onMediaPeriodCreated(WINDOW_INDEX, MEDIA_PERIOD_ID);
+
+    // Remove it a second time and confirm the events stop being propagated.
+    eventDispatcher.removeEventListener(mediaSourceEventListener, MediaSourceEventListener.class);
+
+    verifyNoMoreInteractions(mediaSourceEventListener);
+  }
+
   private interface MediaAndDrmEventListener
       extends MediaSourceEventListener, DrmSessionEventListener {}
 }