You can use alby extension on firefox mobile and connect to web clients. But i don't think it's working in apps yet
Discussion
do you know if is this even a problem?
im assuming if you are using a foss client it can be verified how the client stores your key and make sure it doesnt leave your device. or is it the OS (android and ios) that we are worried about?
is a app worried ... like i say in other comment ... you have amber: