Use system's lock screen instead of custom password

[CampelloManuel]

We lock notes with a password, which is of questionable effectiveness, as noted here #557

Remove password related code and use the system's lock screen (PIN, password, fingerprint, face ID, ...) to unlock the note.

Pros:

• we get a note-locking system that actually works
• password management is delegated to the OS

Cons:

• the user must enable a system lock screen to lock our notes
• the user can not set a different password for this app, so everyone who can use the phone, can also read protected notes