[TIMOB-18856] Windows: hardware back button should close Ti.Window

All Titanium app for Windows Store should follow Windows Store policies described at https://msdn.microsoft.com/en-us/library/windows/apps/dn764944 . This includes app navigation policy on how to handle back button with your page/dialog. Currently Ti app closes immediately when you press the back button even when you open multiple Ti.Window. {quote} Where applicable, pressing the back button should take the user to a previous page/dialog. If the user presses the back button on the first page of the app, then the app terminates (unless it is allowed to run in the background). {quote}