Fix DASH duration.

As multi period DASH isn't supported yet use the duration of the first period only.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127086808

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

@@ -277,7 +277,7 @@ public final class DashMediaSource implements MediaPeriod, MediaSource,
       manifestUri = manifest.location;
       manifestUri = manifest.location;
     }
     }
     if (!prepared) {
     if (!prepared) {
-      durationUs = manifest.dynamic ? C.UNSET_TIME_US : manifest.duration * 1000;
+      durationUs = manifest.dynamic ? C.UNSET_TIME_US : manifest.getPeriodDuration(0) * 1000;
       buildTrackGroups(manifest);
       buildTrackGroups(manifest);
       if (manifest.utcTiming != null) {
       if (manifest.utcTiming != null) {
         resolveUtcTimingElement(manifest.utcTiming);
         resolveUtcTimingElement(manifest.utcTiming);