This is a technical  limitation that Android application cannot obtain keyboard event for ALT+TAB. To push a window to foreground, one way is to use the unityTouch sidebar, open the sidebar, and in the "running apps" section, clicking on an item would push the corresponding remote window to foreground.

Perhaps Horizon Client for Chrome from Chrome Webstore could work for this ALT+TAB requirement. It is worse a try if ALT+TAB matters for you.