How to make keyup on mobile devices?
//This is because your Android has no real keyboard attached,
//it has a virtual one and it does not fire key- events.
//Since you're working with <input>, listen for input event instead of keydown or keyup.
//It solves your problem and also helps to handle paste.
document.getElementById("FirstName").addEventListener("input", alphaOnly, false);
document.getElementById("Surname").addEventListener("input", alphaOnly, false);
function alphaOnly(event) {
event.target.value = event.target.value.replace(/[^a-z]/ig, '');
}
<input id="FirstName"><br>
<input id="Surname">