[native] Don't overwrite main SQLCipher key when creating logs key

Summary: This resolves ENG-6826. The issue is described in this comment.

Test Plan:
I tested upgrading from 314 -> 316 on my Android device. Prior to this diff, it would always result in the bug. After this diff, the bug no longer occurred.

In addition, I tested upgrading from 314 -> master in the same way.

Reviewers: michal, kamil, atul, varun, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D11137