Allow setting biometric advanced unlock after database has been unlocked

**Is your feature request related to a problem? Please describe.**
I want to use biometric advanced unlock but sometimes I forget to press the button. At this point, to enable biometric unlock I need to lock the database, enter the password again, press the button.

**Describe the solution you'd like**
I would like to be able to enable biometric advanced unlock when the database is already unlocked. When the database is unlocked KeePassDX can save it so it seems it must have access to the secret key just like when I enter the password but haven't unlocked the database yet.

No files, tests, or entry points are named. Start by tracing the existing biometric advanced unlock flow and the path used when an already-unlocked database is saved; done means the setting can be enabled without locking and re-entering the password.

Written by the indexing model from the issue text.