Update period index in DashMediaPeriod event dispatcher after manifest update.

Issue:#4492

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205636634

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaPeriod.java

@@ -61,7 +61,6 @@ import java.util.List;
   /* package */ final int id;
   /* package */ final int id;
   private final DashChunkSource.Factory chunkSourceFactory;
   private final DashChunkSource.Factory chunkSourceFactory;
   private final int minLoadableRetryCount;
   private final int minLoadableRetryCount;
-  private final EventDispatcher eventDispatcher;
   private final long elapsedRealtimeOffset;
   private final long elapsedRealtimeOffset;
   private final LoaderErrorThrower manifestLoaderErrorThrower;
   private final LoaderErrorThrower manifestLoaderErrorThrower;
   private final Allocator allocator;
   private final Allocator allocator;
@@ -72,6 +71,7 @@ import java.util.List;
   private final IdentityHashMap<ChunkSampleStream<DashChunkSource>, PlayerTrackEmsgHandler>
   private final IdentityHashMap<ChunkSampleStream<DashChunkSource>, PlayerTrackEmsgHandler>
       trackEmsgHandlerBySampleStream;
       trackEmsgHandlerBySampleStream;
 
 
+  private EventDispatcher eventDispatcher;
   private @Nullable Callback callback;
   private @Nullable Callback callback;
   private ChunkSampleStream<DashChunkSource>[] sampleStreams;
   private ChunkSampleStream<DashChunkSource>[] sampleStreams;
   private EventSampleStream[] eventSampleStreams;
   private EventSampleStream[] eventSampleStreams;
@@ -126,6 +126,13 @@ import java.util.List;
    */
    */
   public void updateManifest(DashManifest manifest, int periodIndex) {
   public void updateManifest(DashManifest manifest, int periodIndex) {
     this.manifest = manifest;
     this.manifest = manifest;
+    if (this.periodIndex != periodIndex) {
+      eventDispatcher =
+          eventDispatcher.withParameters(
+              /* windowIndex= */ 0,
+              eventDispatcher.mediaPeriodId.copyWithPeriodIndex(periodIndex),
+              manifest.getPeriod(periodIndex).startMs);
+    }
     this.periodIndex = periodIndex;
     this.periodIndex = periodIndex;
     playerEmsgHandler.updateManifest(manifest);
     playerEmsgHandler.updateManifest(manifest);
     if (sampleStreams != null) {
     if (sampleStreams != null) {