Implement encryption-decryption layer for elementary file operations

Summary: Implements encryption-decryption layer for flat-file message store operations.

Test Plan: Modify AppDelegate so that on AppLaunch it encrypts and decrypts some text then assert they are equal.

Reviewers: palys-swm, varun, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4384