feat: Close popups and flyout on hardware back button

[MartinZikmund]

GitHub Issue: closes #20757

PR Type:

What is the current behavior? 🤔

What is the new behavior? 🚀

PR Checklist ✅

Please check if your PR fulfills the following requirements:

• 📝 Commits must be following the Conventional Commits specification.
• 🧪 Added Runtime tests, UI tests, or a manual test sample for the changes have been added (for bug fixes / features) (if applicable)
• 📚 Docs have been added/updated which fit documentation template (for bug fixes / features)
• 🖼️ Validated PR Screenshots Compare Test Run results.
• ❗ Contains NO breaking changes

Other information ℹ️

[Copilot]

Pull Request Overview

This PR introduces functionality to handle hardware back button events by closing popups and flyouts. Key changes include:

• Refactoring the singleton pattern in SystemNavigationManager.
• Adding BackButtonIntegration for managing back button listeners.
• Creating runtime tests for back button integration.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Uno.UWP/UI/Core/SystemNavigationManager.cs	Simplified singleton implementation via internal Instance property.
src/Uno.UI/UI/Xaml/Application.cs	Added initialization for back button integration.
src/Uno.UI/DirectUI/BackButton/IBackButtonListener.cs	Introduced new interface for back button listeners.
src/Uno.UI/DirectUI/BackButton/BackButtonIntegration.cs	Implemented listener registration and cleanup using a weak reference pool.
src/Uno.UI.RuntimeTests/Tests/DirectUI/Given_BackButtonIntegration.cs	Added tests to verify back button listener behavior.

Comments suppressed due to low confidence (2)

src/Uno.UWP/UI/Core/SystemNavigationManager.cs:9

• The simplified singleton implementation looks clear and concise; please ensure that changing the visibility to 'internal' is intentional and does not expose the instance beyond its intended scope.

internal static SystemNavigationManager Instance { get; } = new();

src/Uno.UI/DirectUI/BackButton/BackButtonIntegration.cs:45

• [nitpick] Consider verifying that the weak reference pooling mechanism and removal pattern here meet the application's concurrency requirements, even if it's currently used on the UI thread.

WeakReferencePool.ReturnWeakReference(_listeners, weakListener);

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