Re-write the UI...
"The re-write is in the UI, with devs keeping the core of the database, back-end and workflows. Xamarin claims up to 70 per cent code re-use."
With the advent of Xamarin forms, the figure for code re-use is a lot higher now - 90% plus was the claim at a seminar I went to recently.
No idea how accurate this is - I don't write apps. But for games, the only real platform-specific code I've had to write to date are the wrappers that get the apps started on each OS, and lifecycle event handling code.