App lock for extra privacy

- New optional app lock: choose Medium security in Settings > Security to require your device unlock (fingerprint or face if set up, otherwise your PIN, password or pattern) before the app opens
- Prompts on launch and again if you have been away for over a minute; quick app switches stay unlocked
- Hides your data from the app switcher preview while the lock is on
- Medium is only offered when your device has a screen lock set; the default remains no lock
