[native][keyserver] Persist signedIdentityKeysBlob on register from native

Summary:
Ensure that signedIdentityKeysBlob is persisted in the cookies table when user registers from native.

Depends on D6915

Test Plan:
Set breakpoints and observed that cookies table was as expected:

Reviewers: ashoat, tomek, varun, jon

Reviewed By: varun

Differential Revision: https://phab.comm.dev/D6916