OpenReadspeakerBlock: refactor $request_path generation along with template improvements

The current $request_path generation relies on global $base_url.
This approach breaks with the OOP and DI pattern of Drupal.

Current code:

global $base_url;
$request_path = urlencode($base_url . $this->currentPath->getPath());

// ...

$build['open_readspeaker_block'] = [
      '#theme' => 'open_readspeaker_ui',
      '#request_path' => $request_path,
];

We could simply replace that with Url::fromRoute()

$build['open_readspeaker_block'] = [
      '#theme' => 'open_readspeaker_ui',
      '#request_path' => urlencode(Url::fromRoute('<current>', [], ['absolute' => TRUE])->toString()),
];

In addition to the new code we could remove the DI of the $container->get('path.current') service.

the result is the same as before, but without any "magic" global variable.

Any thoughts?