Fix notification Group property being overwritten in unpackaged apps

[andrewleader]

Fixes #3835

For unpackaged apps, we were accidently overwriting the Group property on toasts if developers were setting that. Thus, scenarios like #3835 where a progress bar toast is using both group and tag, updating the progress bar didn't work

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

Group property gets overwritten

What is the new behavior?

Group property only gets written if developers didn't specify it

PR Checklist

Verified via...

• Sample app modified with both using group and not using group for progress bar (and for removing toasts)
• Final NuGet package in separate sample app (pending a build)

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• 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)
• Contains NO breaking changes

Other information

[ghost]

Thanks andrewleader 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 🙌

[michael-hawker]

Seems straight-forward enough. @azchohfi @RosarioPulella want to take a look?

[ghost]

Hello @michael-hawker!

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