Move 'install_time' state update from \Drupal\Core\Installer\Form\SiteConfigureForm to installed_finished()

Problem/Motivation

As uncovered at #2350711: Update module fails on installation → , it's sort of fragile and weird for us to be setting the 'install_time' state only in \Drupal\Core\Installer\Form\SiteConfigureForm::submitForm(). It should really happen in install_finished(). That way, if folks alter the installer form, or use drush or other means to install a site, they'll still correctly have this state variable set.

Once we do this, \Drupal\Core\Installer\Form\SiteConfigureForm will no longer be using the state service at all, so we also need to deprecate that service from the form.

Proposed resolution

• Move the call to set 'install_time' state from SiteConfigureForm::submitForm() to install_finished().
• Deprecate the state service from \Drupal\Core\Installer\Form\SiteConfigureForm.

Remaining tasks

4. Nope, see #15 - phew! 😉
5. #3159456: Remove deprecation and fix constructor args for \Drupal\Core\Installer\Form\SiteConfigureForm →
8. Commit.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A