Fix XamlRoot being set incorrectly in Popup Handler

[ne0rrmatrix]

Description of Change

This fixes incorrect XamlRoot being set when application uses modal navigation and opens a popup. The current behavior results in application crashing. This sets the XamlRoot correctly and the app does not crash. Additionally made a cosmetic size change to sizing for large popup as it is too large and covers the whole window in some situations in Multiple Popups Page.

Linked Issues

• Fixes [BUG] Popup is not working on Windows whe opend from a modal #2459

PR Checklist

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

Additional information

Currently the windows implementation of Popup has the xaml root set incorrectly. I updated the XamlRoot in the handler for Popup to fix the issue. The linked issue has a CTD when using modal navigation and opening a Popup.