Expose isUsingKeystore method from types using Tink encryption under the hood

[mottljan]

When Android Keystore is not possible to be used on certain devices, Tink just silently does not use it and stores data encryption keys in plain text instead of encrypting it using a master key. Tink provides a method AndroidKeysetManager.isUsingKeystore that informs about usage of the Keystore. It would be a good idea to expose this method from types using Tink: EncryptedFile, EncryptedSharedPreferences and both encrypted DataStore types, so the client can use it and react appropriately if needed.