Add methods to securely store a wallet key

[TimoGlastra]

We currently rely on React Native Keychain. We also looked into Expo Secure Store but this doesn't correctly handle the security.

Android now supports EncryptedSharedPreferences, with integrates with the key store. It would be quite straightforward to add a method to store / retrieve a key, which is bound to biometrics. This can be used as a fallback to a PIN.

for iOS the approach from RN Keychain / Expo Secure Store seems sufficient.