Add Focus propagation to MauiView

[PureWeen]

Description

Currently if a MauiView is made focusable we don't propagate the focused properties up to the xplat layer

There's a few layers of conversation here about if we should be setting CanBeFocusable or CanBecomeFirstResponder as well

But, in the mean time it is correct to propagate DidUpdateFocus into the xplat layer in order to be better citizens

I have an example here

https://github.com/davidortinau/Plugin.Maui.KeyListener/blob/c1c112e8250418070d6ee01e8bdc20e2e9fe4636/samples/Plugin.Maui.KeyListener.Sample/FocusableContentViewPlatform.cs#L23-L43