While when you keep the app on your device, the wallet is stored on the device's internal storage encrypted, which I believe is safer than having to type your wallet seed every time you need to make a transaction.
I might be wrong, though!
Wallet file will be encrypted if the wallet is created using a password, and you are right,t though, it makes the chances higher but nothing is absolutely safe in a device which is connected to the internet. We can never be 100% sure that our device is free from any malware or spyware so that we can say I successfully deleted the wallet completely. what if the device screencapture and sends it to somewhere?
The safe option is to create in a mobile that won't be connected to the internet ever.