When resume method promise resolves we are certain that audio context has been unlocked and that we can remove touch event listeners

remove-touch-event-listeners-when-resume-promise-resolves.js

if (context.state === 'suspended' && 'ontouchstart' in window)
{
    var unlock = function()
    {
        context.resume().then(function()
        {
            document.body.removeEventListener('touchstart', unlock);
            document.body.removeEventListener('touchend', unlock);
        });
    };

    document.body.addEventListener('touchstart', unlock, false);
    document.body.addEventListener('touchend', unlock, false);
}

I’ve created an npm package called web-audio-touch-unlock
that exposes this method and makes it easy to use in your own projects.
Just pass your AudioContext instance to it and you’re good to go!

You can try it out yourself here.

Cheers!