Add open Keychain symmetric key to encrypt & decrypt database

Open Keychain : https://github.com/open-keychain/open-keychain

Symmetric encryption is a type of encryption where only one key (a secret key) is used to both encrypt and decrypt electronic information. The entities communicating via symmetric encryption must exchange the key so that it can be used in the decryption process.

Start by reviewing the Open Keychain project linked in the issue and the existing database encryption and decryption flow in KeePassDX. The issue does not name files, tests, or a precise integration point; done would require a defined design for using an Open Keychain symmetric key for both database encryption and decryption.

Written by the indexing model from the issue text.