Fix: Fixed grip splitter cursor behavior for preview pane and secondary pane

[gave92]

Resolved / Related Issues

• Were these changes approved in an issue or discussion with the project maintainers? In order to prevent extra work, feature requests and changes to the codebase must be approved before the pull request will be reviewed. This prevents extra work for the contributors and maintainers.
  Closes Bug: Grid splitter shows wrong cursor #12046

Also simplifies a bit the logic for changing the grid splitter cursor for DetailsLayoutBrowser and MainPage.

Validation
How did you test these changes?

• Did you build the app and test your changes?
• Did you check for accessibility? You can use Accessibility Insights for this.
• Did you remove any strings from the en-us resource file?
  • Did you search the solution to see if the string is still being used?
• Did you implement any design changes to an existing feature?
  • Was this change approved?
• Are there any other steps that were used to validate these changes?
  1. Check cursor behaviour when hovering over the pane splitter
  2. Check cursor behaviour when hovering over the preview pane splitter

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[ferrariofilippo]

LGTM and works for me