App locking timeout/grace period

### Is your feature request related to a problem? Please describe.
When "Lock" is enabled I'd like a longer timeout/grace period before having to re-enter the PIN (or whichever credential is set). I find myself switching between the nextcloud client and another app on my phone regularly and I have to unlock it every time I re-open it.

This might be if I'm downloading or re-arranging photos in folders within nextcloud and I then switch back to my external gallery app to view the changes. I'll have to keep re-entering the PIN when I go back to nextcloud.

Another scenario is when I'm moving data within nextcloud and because it can be really slow I'll switch to another app momentarily whilst waiting.

### Describe the solution you'd like
Either a) a longer timeout before the app asks you to re-enter your PIN or b) the option presented to the user to choose from a preset list (1 minute, 5 minutes, 15 minutes) with the default set to whatever it is currently.

I'd guess some form of b) will be preferable if at all because some users will worry about security more than others.