ThemeListener Refactor & Disposal

[XAML-Knight]

Fixes #3555

The ThemeListener Sample Page uses it's own ThemeListener, when in reality it could just use the Sample App's Sample Controller ThemeListener instance, instead. This PR also includes some extra disposal/clean-up, when navigating away from the ThemeListener sample page.

PR Type

What kind of change does this PR introduce?

Refactoring (no functional changes, no api changes) Sample app changes

What is the current behavior?

ThemeListener Sample Page uses it's own, extra copy of a ThemeListener, and also does not clean-up after itself (Bad 🐶 !).

What is the new behavior?

ThemeListener Sample Page now utilizes the Sample App's Sample Controller instance of a ThemeListener. ThemeListener Sample Page now also implements some clean-up, in the override of OnNavigatedFrom event.

PR Checklist

Please check if your PR fulfills the following requirements:

• [ ] Tested code with current supported SDKs
• [ ] New component
  • [ ] Pull Request has been submitted to the documentation repository instructions. Link:
  • [ ] Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
  • [ ] If control, added to Visual Studio Design project
• [x] Sample in sample app has been added / updated (for bug fixes / features)
  • [ ] Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• [ ] New major technical changes in the toolkit have or will be added to the Wiki e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
• [ ] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [ ] Header has been added to all new source files (run build/UpdateHeaders.bat)
• [x] Contains NO breaking changes

Other information

[ghost]

Thanks XAML-Knight for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌