[Avatar] Add presentational avatar

[danbrady]

WHY are these changes introduced?

Update Avatar to allow its accessibility role to be presentational. This is ideal when the Avatar is supported by content that gives it context. (Ex. In a Users list when a name is supplied, the Avatar can be presentational and does not need a label.)

Fixes #1925

WHAT is this pull request doing?

Avatar detects if an accessibilityLabel, name, or initials are passed and uses that to create an Avatar with a corresponding label and img role. If none of those props are passed, an i18n string "Avatar" is used as the label and continues to use the img role. This PR removes that "Avatar" accessibility label, and the role becomes presentation.

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

import React from 'react';
import {Page, Avatar} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      <Avatar />
      <Avatar name="Hello World" />
      <Avatar initials="DB" />
      <Avatar accessibilityLabel="Hello World" />
    </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

[chloerice]

Thank you for fixing this @danbrady!! 🎉