Fix: font not updating on Android

[mathjeff]

Previously on Android, calling GetTextSize twice with two different fonts would reuse the first font in the second call

This change should cause the font name to be used for each call to GetTextSize

I haven't yet managed to build this project and verify this change (it seems that it might require updating Xamarin), would it help if I did that?

[rudyryk]

Thanks @mathjeff!