Finding ways to protect an app idea before it gets copied is one of the most common mobile app questions we come across. Truth is, you will have the control on your app idea till it is launched in the market. Once deployed, the second your competitors see the positive reviews, they will try and implement the idea in their mobile application as well, until and unless you have patented your app idea, which again works only on some levels.
Meanwhile, while you are at the ideation cum conceptualization stage, you can protect your idea from getting shared by signing an NDA with the app development agencies you are in talks with.
At the end of the day, the only way to protect your app idea is to act on it before your competitors do.