[ToastManager] Move aria-live to containing div for a11y using NVDA

[dleroux]

WHY are these changes introduced?

Fixes #1215

WHAT is this pull request doing?

In order to announce the Toast using the aria-live attribut, NVDA require the element with the aria-live prop to be on screen when the content changes.

Our Toasts are wrapped in a Toast manager. This pr moves the aria-live attribute from the Toast to the Toast Manager

How to 🎩

Using NVDA (on windows) navigate to:

http://localhost:6006/?path=/story/all-components-toast--basic-toast

and ensure that the Toast messages are being announced.

Copy-paste this code in playground/Playground.tsx:

import * as React from 'react';
import {Page} from '@shopify/polaris';

interface State {}

export default class Playground extends React.Component<never, State> {
  render() {
    return (
      <Page title="Playground">
        {/* Add the code you want to test here */}
      </Page>
    );
  }
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide

[dleroux]

@dpersing I wasn't able to get the sound working on NVDA but the tools that write out the content seemed to be working as expected. Can you 🎩 and make sure this does fix our issue?

[yourpalsonja]

Code looks good to me!