Finish PlayerActivity if cleartext not permitted

This is inline with other show-toast-on-error cases in this method, and avoids
leaving the user on a completely black screen.

Note that the maybeRequestReadExternalStoragePermission is an exception because
it's not actually an error case.

#minor-release

PiperOrigin-RevId: 343264869

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

@@ -325,6 +325,7 @@ public class PlayerActivity extends AppCompatActivity
 
       if (!Util.checkCleartextTrafficPermitted(mediaItem)) {
         showToast(R.string.error_cleartext_not_permitted);
+        finish();
         return Collections.emptyList();
       }
       if (Util.maybeRequestReadExternalStoragePermission(/* activity= */ this, mediaItem)) {