Add DomainVerificationPersistence

Contains the XML serialization/parsing code for system state related
to domain verification.

Includes a change to Pair to support Kotlin deconstructing declarations.

Exempt-From-Owner-Approval: Already approved by owners on main branch

Bug: 163565712

Test: atest DomainVerificationPersistenceTest

Change-Id: I4a3e03e9dfc33b4157e0505900c00c37be823ecd