Keep ExtractorMediaSource duration for repreparation.

This allows to know the duration immediately and fixes the temporary unknown
duration in the UI when repreparing the same extractor media source.

The isSeekable property is still reset to false as we can't actually seek
until the seek map has been reloaded.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188145860

library/core/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaSource.java

@@ -319,11 +319,12 @@ public final class ExtractorMediaSource extends BaseMediaSource
     this.eventDispatcher = new EventDispatcher(eventHandler, eventListener);
     this.customCacheKey = customCacheKey;
     this.continueLoadingCheckIntervalBytes = continueLoadingCheckIntervalBytes;
+    this.timelineDurationUs = C.TIME_UNSET;
   }
 
   @Override
   public void prepareSourceInternal(ExoPlayer player, boolean isTopLevelSource) {
-    notifySourceInfoRefreshed(C.TIME_UNSET, false);
+    notifySourceInfoRefreshed(timelineDurationUs, /* isSeekable= */ false);
   }
 
   @Override