[native] Handler for migrating to signed device lists

Summary:
Address ENG-9618.

Code handling migrating user from old flows to signed device lists

Test Plan:
Manual testing - using console logs I confirmed that:

• migration code is run only when latest backup info is absent, and latest device list was unsigned
  • device list is being reordered and signed device list update is sent during migration
  • userkeys backup is created
• normal backup creation runs otherwise

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14148