Adding strings for expand and collapsed for stackable snackbar

[pranjal-glowingstar]

Problem

Root cause

Fix

Validations

(how the change was tested, including both manual and automated tests)

Screenshots

Before	After
Screenshot or description before this change	Screenshot or description with this change

Pull request checklist

This PR has considered:

• Light and Dark appearances
• Automated Tests
• Documentation and demo app examples
• VoiceOver and Keyboard Accessibility
• Internationalization and RTL layouts
• Size classes and window sizes (notched devices, multitasking, different window sizes, etc)

[Copilot]

Pull request overview

Adds new accessibility announcement strings and updates StackableSnackbar behavior to announce expanded/collapsed state and manage focus when the stack expands/collapses.

Changes:

• Added expanded_announcement / collapsed_announcement string resources.
• Announce expand/collapse via View.announceForAccessibility when state.expanded changes.
• Introduced focus management to move focus onto a selected snackbar when expanding, and added per-item focus/semantics behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
fluentui_notification/src/main/res/values/strings.xml	Adds new string resources used for expand/collapse accessibility announcements.
fluentui_notification/src/main/java/com/microsoft/fluentui/tokenized/notification/StackableSnackbar.kt	Adds expand/collapse announcements, introduces focus selection/request logic, and updates semantics/focus modifiers on snackbar rows.

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