[TIMOB-2137] iOS: Opening a modal window over another modal causes issues

Demonstrated in the following code:

var win = Ti.UI.createWindow({backgroundColor:'white'});
var modal1 = Ti.UI.createWindow({backgroundColor:'blue', modal:true});
var modal2 = Ti.UI.createWindow({backgroundColor:'red', modal:true});

var open1 = Ti.UI.createButton({
    title:'Open modal1',
    width:200,
    height:40
});
open1.addEventListener('click', function() {
    modal1.open();
});
win.add(open1);

var open2 = Ti.UI.createButton({
    title:'Open modal2',
    width:200,
    height:40
});
open2.addEventListener('click', function() {
    modal2.open();
});
modal1.add(open2);

var close1 = Ti.UI.createButton({
    title:'Close modal1',
    top:40,
    width:200,
    height:40
});
close1.addEventListener('click', function() {
    modal1.close();
});
modal2.add(close1);

var close2 = Ti.UI.createButton({
    title:'Close modal2',
    top:100,
    width:200,
    height:40
});
close2.addEventListener('click', function() {
    modal2.close();
});
modal2.add(close2);

win.open();