Ancestral restore versioning for PackageManagerBackupAgent

Introduced usage of ANCESTRAL_RECORD_KEY, based on which value
the restore set can be parsed in multiple ways by different
RestoreDataConsumers.

Test: manual - D2D and cloud for different scenarios (O->P, P->P, Q->P)
Test: O->P and P->P will be covered by our automatic e2e tests
Test: a CTS test will be added to verify that backups of PMBA
      generated on Android P+ always contain the ANCESTRAL_RECORD_KEY,
      and it's always the first key
Bug: 64988620
Change-Id: I4634f403da4aefdeb09dc5ca621c4ac349583251