AFAIK, the filesystem is encrypted on any modern device. An attacker would have to first break into the device and then break into the password database. And if they can break into the device it is game over already.

Also, the attacker would have to be really motivated to brute force into it, and if it happened that my phone got stolen and I was worried about your scenario, I'd be just rotating all the high-value passwords I have, which is something that I think can be done quickly.