libsnapshot: Prepare removal of legacy snapshot

During OTA install, when vendor partition is on Android S, add a new flag in SnapshotUpdateStatus file to indicate that we need to support dm-snapshot based snapshot process. This will be used only during post OTA reboot.

The primary change here is the OTA install path. Earlier, dm-snapshot based approach was used; with this change, since both "snapuserd" and "update-engine" resides on system partition, OTA installation will use the userspace snapshot approach.

To maintain backward compatibility, the new flag "legacy_snapuserd" is used only after OTA reboot. This flag will make sure that update-engine will take the dm-snapshot based approach post reboot and for the entire duration of snapshot-merge.

Additionally, during first-stage init if the vendor is on Android S, then "snapuserd" binary will continue to work based off dm-snapshot as none of this change will impact the mount process.

Bug: 304829384
Test: OTA on Pixel
th - OTA from Android S to TOT with vendor on S
Change-Id: Idd9a60b09417cee141b2810e2d4b35e91c845a5c
Signed-off-by: Akilesh Kailash <akailash@google.com>