Fix resume position if user seeks whilst in error state

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146001900

demo/src/main/java/com/google/android/exoplayer2/demo/PlayerActivity.java

@@ -425,7 +425,12 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
 
   @Override
   public void onPositionDiscontinuity() {
-    // Do nothing.
+    if (playerNeedsSource) {
+      // This will only occur if the user has performed a seek whilst in the error state. Update the
+      // resume position so that if the user then retries, playback will resume from the position to
+      // which they seeked.
+      updateResumePosition();
+    }
   }
 
   @Override