locksettings: only log FRP migration when actually done

Instead of logging "Migrated migrated_frp" when the FRP credential
migration is considered, which effectively means whenever the device
boots up for the first time (regardless of whether the migration
actually needed to be done or not), let's only log if the FRP credential
migration is actually being done.  Also make the message clearer.

Bug: 268526331
Change-Id: I8a46c90902da982eb684e49fb4afee19387621c3