Save/restore current cues in SimpleExoPlayer

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=197531839

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

@@ -92,7 +92,7 @@ public class SimpleExoPlayer implements ExoPlayer, Player.VideoComponent, Player
   private AudioAttributes audioAttributes;
   private float audioVolume;
   private MediaSource mediaSource;
-  private @Nullable List<Cue> currentCues;
+  private List<Cue> currentCues;
 
   /**
    * @param renderersFactory A factory for creating {@link Renderer}s to be used by the instance.
@@ -178,6 +178,7 @@ public class SimpleExoPlayer implements ExoPlayer, Player.VideoComponent, Player
     audioSessionId = C.AUDIO_SESSION_ID_UNSET;
     audioAttributes = AudioAttributes.DEFAULT;
     videoScalingMode = C.VIDEO_SCALING_MODE_DEFAULT;
+    currentCues = Collections.emptyList();
 
     // Build the player and associated objects.
     player = createExoPlayerImpl(renderers, trackSelector, loadControl, clock);
@@ -503,7 +504,7 @@ public class SimpleExoPlayer implements ExoPlayer, Player.VideoComponent, Player
 
   @Override
   public void addTextOutput(TextOutput listener) {
-    if(currentCues != null) {
+    if (!currentCues.isEmpty()) {
       listener.onCues(currentCues);
     }
     textOutputs.add(listener);
@@ -779,7 +780,7 @@ public class SimpleExoPlayer implements ExoPlayer, Player.VideoComponent, Player
       mediaSource = null;
       analyticsCollector.resetForNewMediaSource();
     }
-    currentCues = null;
+    currentCues = Collections.emptyList();
   }
 
   @Override
@@ -795,7 +796,7 @@ public class SimpleExoPlayer implements ExoPlayer, Player.VideoComponent, Player
     if (mediaSource != null) {
       mediaSource.removeEventListener(analyticsCollector);
     }
-    currentCues = null;
+    currentCues = Collections.emptyList();
   }
 
   @Override