fix(material/snack-bar): creates default assertive aria-live if not specified #29815
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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