Accessibility: ajax wizard forms do not consistently utilize Drupal.announce | Sessions generated for anonymous users?

Problem/Motivation

The initial ajax response (when clicking 'Next' the first time) doesn't seem to contain an AnnounceCommand. It also generates a PHP session for the anonymous user. This is troublesome because users with open sessions (SESS* cookie) will normally bypass CDNs and other full page caching systems (can happen if the user navigates away from a form when they have a session).

I'm thinking this might be a two-birds-with-one-stone issue.

• Ajax wizard forms do not seem to consistently utilize Drupal.announce as expected.
• Every other step through a wizard form as an anonymous user generates a PHP session for said user.

Steps to reproduce

What I would expect to happen is:

1. Load up an ajax wizard form.
2. Assert that #drupal-live-announce is empty
3. Click 'Next'
4. Assert that #drupal-live-announce now contains "Test: Webform: Wizard basic: Step 2" loaded. (2 of 4)
5. Click 'Previous
6. Assert that #drupal-live-announce now contains "Test: Webform: Wizard basic: Step 1" loaded. (1 of 4)

What really seems to happen is:

1. Load up an ajax wizard form.
2. #drupal-live-announce is empty
3. Click 'Next'
4. #drupal-live-announce is empty
5. Click 'Previous
6. #drupal-live-announce now contains
   

   "Test: Webform: Wizard basic: Step 2" loaded. (2 of 4)
   "Test: Webform: Wizard basic: Step 1" loaded. (1 of 4)

Proposed resolution

I think the path forward here might involve not storing the announcement in the user's session, but rather in memory (perhaps drupal_static or \Drupal\Core\Cache\MemoryCache\MemoryCache?

Remaining tasks

Make the patch beautiful. The service should probably be injected somehow / somewhere. I wasn't sure what the best way to approach it was because the service is needed within a trait. Procedurally grabbing the service is not an optimal approach.

Does the test require a new test form, or is there an existing one that can/should be used? If an existing form can be used, then there'd be less patch noise.

User interface changes

None

API changes

Perhaps a new service if the memory cache service is preferable, otherwise none if drupal_static is used.

Data model changes

None