liblp: MetadataBuilder::NewForUpdate takes always_keep_source_slot arg

When applying a downgrade package on a Virtual A/B device
(to a non-Virtual A/B build), source slot partitions must
be kept in the metadata.

Test: liblp_test
Bug: 138258570
Change-Id: I87afe68bcfa768bdc015f5966b593758b856c741
2 files changed