WPF/Accessibility: Setting FontSize property prevents system font scaling from working

**Environment:**
- .NET Core 3.1

**Problem description:**
While doing an accessibility pass for FancyZones Editor in PowerToys, we've encountered [this issue](https://github.com/microsoft/PowerToys/issues/10766). I've given a detailed explanation in [this comment](https://github.com/microsoft/PowerToys/issues/10766#issuecomment-924207415), but it boils down to the difference between `FontSize` property behavior in UWP/WPF.

**Actual behavior:**
Setting `FontSize` property in WPF prevents system-wide font scaling setting from working, no matter which [context switch overrides](https://docs.microsoft.com/en-us/dotnet/framework/whats-new/whats-new-in-accessibility#taking-advantage-of-accessibility-enhancements) are enabled.

**Expected behavior:**
Same as with UWP apps: elements' font size changes depending on the scale setting even when `FontSize` is explicitly set for them.

**Minimal repro:**
- create Blank WPF app
- go to `MainWindow.xaml` and add the following xaml inside the `Grid`:
```xaml

```
- launch the application
- Start ->"Make text size bigger" -> Change to e.g. 200%
- Observe that the text size remains the same

Now repeat the steps with a blank UWP app and observe that "Make text size bigger" option affects text size.