Fix content position for postroll ads

The position returned by getContentPosition() could be C.TIME_END_OF_SOURCE.
Fix the content position stored in MediaPeriodInfos for postroll ads to be the
duration of the containing period.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162322339

library/core/src/main/java/com/google/android/exoplayer2/MediaPeriodInfoSequence.java

@@ -223,8 +223,9 @@ import com.google.android.exoplayer2.source.MediaSource.MediaPeriodId;
           || !period.isAdAvailable(adGroupCount - 1, 0)) {
         return null;
       }
+      long contentDurationUs = period.getDurationUs();
       return getMediaPeriodInfoForAd(currentPeriodId.periodIndex, adGroupCount - 1, 0,
-          currentMediaPeriodInfo.endPositionUs);
+          contentDurationUs);
     }
   }