[CryptoModule] store only pickled Olm Account in User Keys Backup

Summary:
Discussed in ENG-9659.

Restoring should work without changes, session set will be empty.

Depends on D14005

Test Plan: Test creating User Keys backup

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14044