[a11y [MatSnackBar] Screen reader announces message twice.

[glushkova91]

What is the expected behavior?

Screen reader should announce snack bar message only once.

What is the current behavior?

Using open API method the message is announced twice because:

• role='status' behaves as aria-live and this role is assigned to massage wrapper
• MatSnackBar uses LiveAnnouncer (or smth similar from cdk a11y module) to announce message.

What are the steps to reproduce?

1. Open shared example
   https://stackblitz.com/angular/krygnaabbpg?file=app%2Fsnack-bar-overview-example.ts
2. Enable SR.
3. Go to "Show Snack-bar" button and press Enter.
4. Observe the result - snack-bar's message is announced twicw

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Chromevox- Google Chrome OS Version 64.0.3282.190

[sandrotonon]

For me it's announced when it's opened and when it's closed (the announcement on close is unnecessary and should be omitted IMHO).

Using ChromeVox and Google Chrome 71.0.3578.98.

[crisbeto]

Should be resolved by #14504.

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