MediaPeriod.getDurationUs must report duration when known.

As per the spec of MediaPeriod.getDurationUs, when a live
stream ends we need to start returning the correct final
duration.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128471183

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

@@ -47,7 +47,6 @@ import java.util.List;
   private final EventDispatcher eventDispatcher;
   private final long elapsedRealtimeOffset;
   private final Loader loader;
-  private final long durationUs;
   private final TrackGroupArray trackGroups;
 
   private ChunkSampleStream<DashChunkSource>[] sampleStreams;
@@ -55,6 +54,7 @@ import java.util.List;
   private Callback callback;
   private Allocator allocator;
   private DashManifest manifest;
+  private long durationUs;
   private int index;
   private Period period;
 
@@ -76,6 +76,7 @@ import java.util.List;
   public void updateManifest(DashManifest manifest, int index) {
     this.manifest = manifest;
     this.index = index;
+    durationUs = manifest.dynamic ? C.UNSET_TIME_US : manifest.getPeriodDuration(index) * 1000;
     period = manifest.getPeriod(index);
     if (sampleStreams != null) {
       for (ChunkSampleStream<DashChunkSource> sampleStream : sampleStreams) {

library/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaPeriod.java

@@ -48,7 +48,6 @@ import java.util.List;
   private final Loader manifestLoader;
   private final int minLoadableRetryCount;
   private final EventDispatcher eventDispatcher;
-  private final long durationUs;
   private final TrackGroupArray trackGroups;
   private final TrackEncryptionBox[] trackEncryptionBoxes;
 
@@ -65,7 +64,6 @@ import java.util.List;
     this.manifestLoader = manifestLoader;
     this.minLoadableRetryCount = minLoadableRetryCount;
     this.eventDispatcher = eventDispatcher;
-    durationUs = manifest.durationUs;
     trackGroups = buildTrackGroups(manifest);
     ProtectionElement protectionElement = manifest.protectionElement;
     if (protectionElement != null) {
@@ -103,7 +101,7 @@ import java.util.List;
 
   @Override
   public long getDurationUs() {
-    return durationUs;
+    return manifest.durationUs;
   }
 
   @Override