Fix a subtle issue with PropertyChanged in ViewModels

[zadjii-msft]

While debugging #13694, we discovered a very subtle bug we had accidentally introduced in a few places. ViewModelHelper defines a PropertyChanged event, backed by a _propertyChangedHandlers event. All opbservable properties in the viewmodels are supposed to run through that event. However, if you do WINRT_CALLBACK(PropertyChanged, Windows::UI::Xaml::Data::PropertyChangedEventHandler) in a derived class, it'll override that original method with the new one. XAML will subscribe to the second one, which is backed by _PropertyChangedHandlers, but the properties will still raise notifications on the callbacks registered to _propertyChangedHandlers.

This change makes it more explicit in these derived classes, that the PropertyChanged method exposed by these classes is indeed the one that's implemented in the base class.

This is a bit of a footgun, for sure. AuditMode would have apparently caught this, as we'd be overriding a method without using the override keyword.

Unblocks #13694.

[DHowett]

@carlos-zamora you will need to propagate some of these changes to the new ColorScheme page!

[DHowett]

I converted this to a draft so that it would not merge without a description

[lhecker]

(This should be "why" and not "what".)

[carlos-zamora]

Once this merges, I'll merge it into #13269 and update that PR accordingly

[zadjii-msft]

@msftbot merge this in 2 minutes

[ghost]

Hello @zadjii-msft!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I won't merge this pull request until after the UTC date Thu, 25 Aug 2022 13:59:01 GMT, which is in 2 minutes

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".