libsnapshot: Require snapuserd in CreateUpdateSnapshots().

Previously, we forced recovery to use traditional dm-snapshot writes in
update_engine. Instead, allow snapuserd in recovery, and forbid
dm-snapshot entirely.

This change also turns down the vab_legacy_test suite.

Bug: 416748567
Test: atest RecoveryOtaTest
Change-Id: Iafa2b756f309c2a5965e03a1e348db3bf0df19be