chore: Fixing reference to UIWindow

[nickrandolph]

GitHub Issue (If applicable): closes #2240

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

UIWindow not found using the old _window field.

What is the new behavior?

v5.2 of Uno adds multi-window support along with refactoring inner logic for Window management.
the internal NativeWindow property now returns the UIWindow required for Maui Embedding

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Updated the Release Notes
• Associated with an issue (GitHub or internal)

Other information

Internal Issue (If applicable):

[nickrandolph]

@Mergifyio backport release/stable/4.1

[mergify]

backport release/stable/4.1

✅ Backports have been created

• #2255 chore: Fixing reference to UIWindow (backport #2246) has been created for branch release/stable/4.1 but encountered conflicts

[MartinZikmund]

If not pressed for time, then this could probably be adjusted to use WindowHelper.GetNativeWindow and ApplicationHelper.Windows see docs here unoplatform/uno#16223