Context: https://github.com/heyThorsten/GCTest
Fixes: dotnet#20023

In testing the above sample, a page with a `RadioButton` has a memory
leak due to the usage of `Border.StrokeShape`.

There was also a slight "rabbit" hole, where we thought there was also
an issue with `GestureRecognizers`. But this was not the case in a real
app (just unit tests):

* dotnet#21089

It turns out that when `GestureRecognizers` are used in `MemoryTests`,
they will leak if there is no `Window` involved.

Instead of using `MemoryTests` like I would traditionally, we should
instead use `NavigationPageTests.DoesNotLeak()`. I can reproduce the
problem with `RadioButton` *and* a `Window` exists in the test.

~~ Underlying issue ~~

It appears that `Border.StrokeShape` does not use the same pattern as
other properties like `Border.Stroke`:

* On set: `SetInheritedBindingContext(visualElement, BindingContext);`
* On unset: `SetInheritedBindingContext(visualElement, null);`

It instead used:

* On set: `AddLogicalChild(visualElement);`
* On unset: `RemoveLogicalChild(visualElement);`

6136a8a that introduced these does not mention why one was used over
another. I am unsure if this will cause a problem, but it fixes the leak.

This makes `Border.StrokeShape` closer to existing code in `Border.Stroke`.