Add an option in DevDiagnostics: Settings where the user can disable the DevDiagnostics startup task

[andreww-msft]

Suggested new feature or improvement

The user can already use the normal process for enabling/disabling a startup task: go to Windows Settings | Apps | Startup apps.

That said, it would be reasonable to include this same ability within PI itself. NOTE: this should be implemented by invoking the Settings | Apps | Startup apps page, using the standard protocol ms-settings:startupapps, and not via any direct manipulation of the startup task in any other way.

Also see related #3465