Use $this->config() where possible

This is a maintenance patch ( For Drupal9! )

Regarding classes that extend FormBase and ControllerBase

We should drop code of of the form \Drupal::config and call the equivalent method provided by the base class.

-    $config = \Drupal::config('adsense.settings');
+    $config = $this->config('adsense.settings');

The patch is small, single minded and should be quick to review.