[identity] Make set_prekey() update devices table too

Summary:
Updated DatabaseClient::set_prekey to also update the devices table with the new prekey (double-writing).

Depends on D10301

Test Plan: Called RefreshUserPreKeys RPC and checked with awslocal dynamodb scan that both users and devices table were updated correctly (with the same prekeys)

Reviewers: varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10302