[client-backup] call C++ function for secure generating random string

Summary:
ENG-4498.

Placing this here to execute this on crypto thread.

Depends on D8886

Test Plan: Call commCoreModule.generateRandomString.

Reviewers: michal, marcin

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8887