UploadMediaForm::submitForm does not honor 'Upload destination'

UploadMediaForm::submitForm has hard coded the public file scheme as the destination:

if (file_exists('public://' . $file['filename'])) {
  $this->logger->notice($this->t('File @filename already exists - It has been replaced', ['@filename' => $file['filename']]));
  drupal_set_message($this->t('File @filename already exists - It has been replaced', ['@filename' => $file['filename']]));
}
$file_entity = file_save_data($data, 'public://' . $file['filename'], FILE_EXISTS_REPLACE);

Drupal comes with a second uri scheme,

private://

and allows additional schemes/stream wrappers to be defined. The file destination is configured in the destination field, so let's use that.

Patch follows.