Adding strings for expand and collapsed for stackable snackbar

[pranjal-glowingstar]

Problem

The talkback was not actively selecting the snackbar items when list was expanded or collapsed. Focus on the items was not handled at all. The announcement of list expanded/collapsed was also not being done.

Root cause

The implementation for talkback announcement for expansion and collapsing was not implemented. Also, the proper criterion for selection of the item was not implemented.

Fix

We have now implemented the entire flow. Talkback will announce list expanded/collapsed whenever the items are expanded/collapsed. Whenever a new element will be added, the focus would directly go to the added element. On removing elements, the focus will not be removed unless the element that is being removed is in the focus. In this case, we will assign the focus to next element in cyclic order. All the items present in the SnackbarItem (title, action text, trailing icon) will be announced in the talkback when selected.

Validations

Validated the build by testing in local and verifying the same.

(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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[Copilot]

Pull request overview

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

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