WinUI/XAML requires the `SelectedItem` to be member of the list of
`ItemsSource`. `CascadiaSettings::DefaultTerminals()` is such an `ItemsSource`
and is called every time the launch settings page is visited.
It calls `DefaultTerminal::Available()` which in turn calls `Refresh()`.
While the `SelectedItem` was cached in `CascadiaSettings`, the value of
`DefaultTerminals()` wasn't. Thus every time the page was visited, it refreshed
the `ItemsSource` list without invalidating the current `SelectedItem`.
This commit prevents such accidental mishaps from occurring in the future,
by moving the responsibility of caching solely to the `CascadiaSettings` class.
## PR Checklist
* [x] Closes#11424
* [x] I work here
* [x] Tests added/passed
## Validation Steps Performed
* Navigating between SUI pages maintains the current dropdown selection ✔️
* Saving the settings saves the correct terminal at `HKCU:\Console\%%Startup` ✔️
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