Handle user un-enrolling device biometrics while biometric lock is enabled

[PhilKes]

Fixes #229

If the biometric lock is enabled, and the app should be locked, but the user un-enrolled all Biometrics/PIN a new dialog appears, asking to the user to either setup biometrics again or to disable biometric lock:

notallyx_issues_229.webm

Notes:

• If NotallyX was unlocked, the user removes all biometrics and then immediately jumps back to NotallyX the new dialog will not appear since the app hasnt been locked in the mean time. The dialog will appear if the user re-starts the app or locks the device again (which require pin or biometrics to be re-setup)
• Disabling biometric lock also means decrypting the database, which requires the encryption key. Usually this encryption key is itself only stored encrypted in the preferences, therefore there is no way to decrypt it without the corresponding BiometricPrompt. To be able to decrypt the database when the user removed all biometrics, the encryption key is also stored as a fallback in EncryptedPreferences when the user initially enables the biometric lock