fix: migrate instant-app private key to the full app
This PR allows the private key generated by the instant-app to be used by the full app when the user installs it.
The idea behind this is that FlutterSecureStorage stores the data in KeyStore, which isn’t shared between the instant-app and the full app. By adding the option to use Encrypted Shared Preferences in FlutterSecureStorage on Android, both the app and the instant-app can use the same private key while maintaining the same security levels.