Views exposed form is never cached in ExposedFormCache

Problem/Motivation

I was looking at some code related to views exposed forms and noticed something weird in Drupal\views\ExposedFormCache.
This class is supposed to set/get the form cache, but setForm method actually does this:

public function setForm($view_id, $display_id, array $form_output) {
  // Save the form output.
  $views_exposed[$view_id][$display_id] = $form_output;
}

As a result, getForm is always return FALSE.

Proposed resolution

Let's make it work.

Remaining tasks

Review, commit.