[TIMOB-11844] Android: preferences.xml: load defaults on start

*Problem description* When you use a preferences.xml file, you would expect its default values to be set upon project creation. So I added a loadPreferences, method, you can use that method to load default values for a preference file. Moreover, upon app start, the default preferences file (xml.preferences) is automatically loaded. *Test case*

var win = Ti.UI.createWindow({
	backgroundColor : 'white'
});

win.addEventListener('open', function() {
	if (Ti.App.Properties.hasProperty('testmeprop')) {
		alert('property testmeprop exists!');
	} else {
		alert('property testmeprop DOES NOT EXIST!');
	}
});
win.open();

I attached the example preferences.xml.