Move CompositionSwitcher to Experimental namespace

[acoates-ms]

This moves all the objects related to the composition switcher into an Experimental namespace.

On public objects that used to expose objects from the switcher we now directly expose Microsoft.UI.Composition objects. There are experimental interfaces that can be used to set composition switcher objects instead.

This should limit the effect of the switcher on the public interface. We still use the switcher internally to allow Office to be able to continue to inject a custom composition interface. But now when we remove this ability APIs outside of the experimental namespace should not need to change.

Example:

Before:

winrt::Microsoft::ReactNative::Composition::CompositionUIService::SetCompositionContext(
    InstanceSettings().Properties(),
    winrt::Microsoft::ReactNative::Composition::MicrosoftCompositionContextHelper::CreateContext(
        g_liftedCompositor));

// Have to use MicrosoftCompositionContextHelper to access actual Microsoft.UI.Composition visual.
auto rootVisual = winrt::Microsoft::ReactNative::Composition::MicrosoftCompositionContextHelper::InnerVisual(compositionRootView.RootVisual());

After:

winrt::Microsoft::ReactNative::Composition::CompositionUIService::SetCompositor(
    InstanceSettings(), g_liftedCompositor);

// CompositionRootView.RootVisual now directly returns a Microsoft.UI.Composition visual.
auto rootVisual = compositionRootView.RootVisual();

Microsoft Reviewers: Open in CodeFlow

[jonthysell]

nit: Maybe just leave in as <UseFabric>false</UseFabric>?