UWP xr session context should have its lifecycle managed by the xr system compared to a static object

[chrisfromwork]

we should add additional checks to not create the underlying xr system if xr isn't supported

[syntheticmagus]

We should try to get this to fall back into Babylon.js's default behaviors for browsers that don't support WebXR, since that should handle the situation gracefully from the JavaScript side in a way that will match the Web.