[native] Fix migration

Summary: cookie field migration on natvie was incorrect - it resulted in kesyerver store looking like: { keyserverInfos: {['256']: {1: 'u', 2: 's', 3: 'e', 4: 'r' ...}}

Test Plan: Tested using redux dev tools that not cookie is migrated correctly and keyserver store looks like: { keyserverInfos: {['256']: {cookie: 'user:...'}}

Reviewers: michal, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8626