More secure biometric implementation

- On iOS, store the newly generated keys in the Secure Enclave. https://developer.apple.com/documentation/security/certificate_key_and_trust_services/keys/protecting_keys_with_the_secure_enclave
- On Android, store the newly generated keys in Strongbox Keymaster, if it is available.
https://developer.android.com/training/articles/keystore#HardwareSecurityModule
- Make sure preexisting keys are still usable. That is, if biometric has been enabled before the update, it must still work.

The update does not effect the public API. To take effect, biometric has to be disabled, and enabled again.

Locate the iOS and Android biometric key-generation and storage paths, then inspect how existing keys are loaded after an update. Check the platform documentation linked in the issue first; done means new keys use Secure Enclave or StrongBox when available, while preexisting keys remain usable after re-enabling biometric authentication.

Written by the indexing model from the issue text.