Fix Windows Media Element in MultiWindow Mode on exit Crash

[ne0rrmatrix]

• Bug fix

Description of Change

Track window count and manage snackbar notifications

Added a static NumberOfWindows property to the Options class to track the number of windows. Updated SetShouldEnableSnackbarOnWindows to increment/decrement NumberOfWindows on window creation/closure. Conditional registration/unregistration of AppNotificationManager for snackbar notifications based on NumberOfWindows count.

Linked Issues

• Fixes #2257
• Fixes #2279

PR Checklist

• [x] Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• [ ] Has tests (if omitted, state reason in description)
• [x] Has samples (if omitted, state reason in description)
• [x] Rebased on top of main at time of PR
• [x] Changes adhere to coding standard
• [ ] Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

This PR uses a simple counter to track the number of windows that exist and only when one is remaining will it unregister Snackview