I want to protect my phone in case of stolen, so i want add an app which will install after factory reset. I have root acces. I want to that app be automatically installed, and factory reset won't remove it. What do i need to do?
Basically what you need to do is to install a custom rom on your phone. Tweak the rom so that your app is included as a system app. If the user does a factory reset, the systems apps go back to their default state.
You'll want to test it out, though. I don't know if a factory reset wipes all the data for system apps (clean slate) or if it resets it to how the app was when the rom was installed.
On top of that, though, a user who steals your phone may install a rom of their own. In that case, you are out of luck. As far as I know, there does not exist a custom recovery that can be password protected (to keep people from installing another rom).