A method of backing up software wallet

I've reset my old android phone to factory settings. I've not connect a SIM-card and Wi-Fi on it. If I install a open-source wallet on it (only once connected via usb), will it be a secure enough backup of the wallet to store it separately from the seed, for example, in a desk at work? If someone steals the phone without knowing about the wallet on it, then probably without a password to the phone (and even more so to the wallet) they will simply reset and clear the internal memory of the phone. Or at least the passwords will give me time to change the compromised wallet. The conversation is not about large sums and very complex "spy" method of backing up. What would you recommend to me?