Add update_verifier for A/B OTA update.

update_verifier checks the integrity of the updated system and vendor
partitions on the first boot post an A/B OTA update. It marks the
current slot as having booted successfully if it passes the verification.

This CL doesn't perform any actual verification work which will be
addressed in follow-up CLs.

Bug: 26039641
Change-Id: Ia5504ed25b799b48b5886c2fc68073a360127f42