[TIMOB-6684] Android: Move Object.prototype.extend to the "kroll" namespace

We currently define Object.prototype.extend for mixin style functionality in platform. Some 3rd party frameworks and modules use the API name "extend" in other places which would potentially conflict with our use. We can easily avoid this conflict by using our internal "kroll" namespace for this API. To test, just put the following in a test app.js and once run value of extend in the log should be "undefined":

var win = Ti.UI.createWindow({backgroundColor: "red"});
win.open();

var x = {};
Ti.API.info("-------> value of extend for \"x\" is: " + x.extend);

if (x.extend === undefined) {
	Ti.API.info("\"x\" is undefined");

} else {
	Ti.API.info("\"x\" is not undefined");
}