Hide subtitles when switching player in SimpleExoPlayerView

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177941993

library/ui/src/main/java/com/google/android/exoplayer2/ui/SimpleExoPlayerView.java

@@ -425,6 +425,9 @@ public final class SimpleExoPlayerView extends FrameLayout {
     if (shutterView != null) {
       shutterView.setVisibility(VISIBLE);
     }
+    if (subtitleView != null) {
+      subtitleView.setCues(null);
+    }
     if (player != null) {
       if (surfaceView instanceof TextureView) {
         player.setVideoTextureView((TextureView) surfaceView);

library/ui/src/main/java/com/google/android/exoplayer2/ui/SubtitleView.java

@@ -19,6 +19,7 @@ import android.annotation.TargetApi;
 import android.content.Context;
 import android.content.res.Resources;
 import android.graphics.Canvas;
+import android.support.annotation.Nullable;
 import android.util.AttributeSet;
 import android.util.TypedValue;
 import android.view.View;
@@ -87,9 +88,9 @@ public final class SubtitleView extends View implements TextOutput {
   /**
    * Sets the cues to be displayed by the view.
    *
-   * @param cues The cues to display.
+   * @param cues The cues to display, or null to clear the cues.
    */
-  public void setCues(List<Cue> cues) {
+  public void setCues(@Nullable List<Cue> cues) {
     if (this.cues == cues) {
       return;
     }