Use consistent identifiers for extractor periods.

Also, fix clearing the timeline for source info changes.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130292700

library/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -821,6 +821,8 @@ import java.io.IOException;
         // Release all loaded periods and seek to the new playing period index.
         releasePeriodsFrom(playingPeriod);
         playingPeriod = null;
+        readingPeriod = null;
+        loadingPeriod = null;
 
         MediaSource.Position defaultStartPosition =
             mediaSource.getDefaultStartPosition(newPlayingPeriodIndex);
@@ -851,6 +853,8 @@ import java.io.IOException;
             index = playingPeriod.index;
             releasePeriodsFrom(playingPeriod);
             playingPeriod = null;
+            readingPeriod = null;
+            loadingPeriod = null;
             seekToPeriodPosition(index, 0);
             return;
           }

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

@@ -135,7 +135,7 @@ public final class ExtractorMediaSource implements MediaSource, MediaSource.List
   @Override
   public void prepareSource(MediaSource.Listener listener) {
     sourceListener = listener;
-    timeline = new SinglePeriodTimeline(this, C.UNSET_TIME_US, false);
+    timeline = new SinglePeriodTimeline(0, C.UNSET_TIME_US, false);
     listener.onSourceInfoRefreshed(timeline, null);
   }