I believe Android has a duress pin starting in version 5 or 6, under advance settings. This is probably ideal for what you're talking about.

Start with prevention though - don't have the newest most expensive phone, don't make yourself look like an easy victim (distracted, alone, scared, weak), don't go to bad neighborhoods if it can be avoided, learn some self defense. If armed robbery is really a pressing concern there are options in many places to meet that threat with similar force. There are some criminals that will kill you even after getting what they want. That was happening in my city a while back, and I'm sure it occasionally still does.