Please give me feed back whether it's good practice to have a back button in activity, or is it just nonsense? Maybe some android phones don't have back/return key? Or do all models have them?
All android phones will have the back key. Putting a back button is, IMO, ugly and unnecessary; you better use your space with useful stuff. Sometimes your boss will just obligate you to put a Back button, usually because (sadly) most of the android sketchs are based on iPhone's ones... so the only reason why I'd put a back button is to not get fired (well, sometimes it's much better getting fired than becoming a poor programmer).