[MacOS] Readonly setting on macos only takes hold once field loses first responder

[PureWeen]

Description

https://github.com/xamarin/Xamarin.Forms/pull/1972/files#r210107776

MacOS looks to have a quirk where if the field is the first responder when it goes readonly the field remains editable until it's no longer the first responder

Another way to see this is to add focus calls here.

toggle.Toggled += (_, b) =>
			{
				entryToggleable.IsReadOnly = b.Value;
				editorToggleable.IsReadOnly = b.Value;

				entryToggleable.Focus();
				editorToggleable.Focus();
			};

I'm wondering if ShouldBeginEditing would be a more thorough way to go?
https://stackoverflow.com/questions/7949071/iphone-ipad-how-to-make-uitextfield-readonly-but-not-disabled

Wire that up and return false if Element.IsReadOnly

Steps to Reproduce

1. run Issue1678.cs and follow the steps in the gif

Expected Behavior

field should become readonly immediately not when you click out of field

Actual Behavior

you can still type in the field until the user clicks out of the field