Add option to Settings enable app protection via default biometrics/swipe/passcode/PIN + a app-managed 6 digit PIN

When user taps to enable in Settings:

```
if enabled
//let user enter a 6 digit PIN mananged by the app
//enter again to confirm
end
```

Upon app launch/resume:

```
if enabled
if biometrics/swipe/passcode/PIN enabled in OS
//Gate with biometrics/swipe/passcode/PIN
else
//Gate with app-managed 6-digit PIN
end
else
//Show app as usual
end
```

This is how it looks like on iOS:

Settings|Set|Confirm|Upon launch/resume
-|-|-|-
|||

Looks like this if entry fails:

First few times|After exceeding the number of attempts
-|-
|

(screenshot should say "passcode" instead of "password")