Nullify PlatformState.[previous]AddressBook fields

[anthony-swirldslabs]

Background
To remove the AB fields from the PlatformState, we need to first migrate any data from them into the RosterService states (unless we've started populating the latter already.)

Acceptance criteria

• Update PlatformService and RosterService migration schemas to migrate the data from PlatformState.[p]AB to the Roster state/map, unless the RosterService states have data already.
• Nullify the PlatformState.[p]AB fields