libsnapshot: Enable snapshots during OTA install for S vendor

ro.virtual_ab.userspace.snapshots.enabled is a vendor property which isn't present in Android S. Hence, during OTA install with S vendor, userspace_snapshots is disabled.

However, both update_engine and snapuserd are already on the system partition during install. Hence, forcefully enable userspace_snapshots if this is a path of legacy dm-snapshots with Vendor on Android S.


Bug: 331156940
Test: OTA tests on treehugger, pixel OTA
Change-Id: I3d1c03493d83e670e37df088d4b676c4aa1dc720
Signed-off-by: Akilesh Kailash <akailash@google.com>