[TIMOB-11802] Android: "android:back" event does not work

Description: "android:back" event does not work on Android, so clicking hardware "back" button does not bring an alert with message "Back button". This is a regression, does not occur on 2.1.4. Steps: 1. Run the following code:

var win = Ti.UI.createWindow();
win.open();

win.addEventListener('android:back', function(){
	alert('Back button');
});
win.addEventListener('android:search', function(){
	alert('Search');
});
win.addEventListener('android:camera', function(){
	alert('Camera');
});

2. Press hardware "back" button Expected result: Should get an alert with message "Back button" Actual result: No alert