Thanks for responding me. So is that option is available in moodle 3.3 ?
Actually my instance is running on moodle 3.3. And another thing is on Android devices and iphone x (ios 11.2.6 ) the app working fine,its keep me logged in.but in iphone 7 and iphone 6 (ios 11.2.6) its giving these logout issue after 4hrs. of inactivity.