Announce callout contents in custom callout test

[ksiler]

Platforms Impacted

• iOS
• macOS
• win32 (Office)
• windows
• android

Description of changes

Fixes an accessibility bug where the contents of the custom callout are not announced/accessible.

Ensures the contents of the custom callout are announced by screen readers by adding the content to the announcement when the callout shows. Also adds a focusable item in the custom callout so keyboard and screen reader users can navigate and close the callout.

Verification

Tested announcement and keyboarding with Narrator.

Before	After
Press for Callout, Button, Be informed that a customized callout has been opened. Customized Callout, Group,	Press for Callout, Button, Be informed that a customized callout has been opened. This is an example message. Customized Callout Group, Close, Button, 1 of 1

Pull request checklist

This PR has considered (when applicable):

• Automated Tests
• Documentation and examples
• Keyboard Accessibility
• Voiceover
• Internationalization and Right-to-left Layouts