fix(material/snack-bar): creates default assertive aria-live if not specified

[essjay05]

Updates Angular Material Snack-Bar component snack-bar-container's aria-live value from AriaLivePoliteness value to default to assertive if there is no given value. This is to fix existing bugs where the snack-bar content is not read by the screenreader when the snack-bar appears.

Fixes b/369493945

[github-actions[bot]]

Deployed dev-app for 7c6cf92a9222cf239c1f37887745f6446c42cfab to: https://ng-dev-previews-comp--pr-angular-components-29815-dev-vw9a3w40.web.app

Note: As new commits are pushed to this pull request, this link is updated after the preview is rebuilt.