Use SQLite with Room for database & Cipher with CWAC-SafeRoom for secure 256-bit AES encryption:
https://developer.android.com/topic/libraries/architecture/room
https://www.zetetic.net/sqlcipher/sqlcipher-for-android/
https://github.com/commonsguy/cwac-saferoom/
The project has been completed and this repo will soon be archived.
But work on the multi-platform NoteDelight application continues.