Ensure the demo app requests local-file permissions for subtitle files

Currently we only do this for the main sample URI, not for subtitles.
So pairing a web-based video file with a local subtitle file on a fresh
install just throws a 'permission denied' exception.

PiperOrigin-RevId: 290794091

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

@@ -436,6 +436,11 @@ public class PlayerActivity extends AppCompatActivity
       }
       Sample.SubtitleInfo subtitleInfo = sample.subtitleInfo;
       if (subtitleInfo != null) {
+        if (Util.maybeRequestReadExternalStoragePermission(
+            /* activity= */ this, subtitleInfo.uri)) {
+          // The player will be reinitialized if the permission is granted.
+          return Collections.emptyList();
+        }
         Format subtitleFormat =
             Format.createTextSampleFormat(
                 /* id= */ null,