Retain playback position on re-preparation

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172736350

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

@@ -443,6 +443,10 @@ import java.io.IOException;
     loadControl.onPrepared();
     if (resetPosition) {
       playbackInfo = new PlaybackInfo(0, C.TIME_UNSET);
+    } else {
+      // The new start position is the current playback position.
+      playbackInfo = new PlaybackInfo(playbackInfo.periodId, playbackInfo.positionUs,
+          playbackInfo.contentPositionUs);
     }
     this.mediaSource = mediaSource;
     mediaSource.prepareSource(player, true, this);