Fix video disappearing when the app is brought out of the background

PiperOrigin-RevId: 489212224

demos/transformer/src/main/java/com/google/android/exoplayer2/transformerdemo/TransformerActivity.java

@@ -190,6 +190,7 @@ public final class TransformerActivity extends AppCompatActivity {
       throw new IllegalStateException(e);
     }
     informationTextView.setText(R.string.transformation_started);
+    progressViewGroup.setVisibility(View.VISIBLE);
     inputCardView.setVisibility(View.GONE);
     outputPlayerView.setVisibility(View.GONE);
     Handler mainHandler = new Handler(getMainLooper());