libsnapshot: OTA upgrade when vendor partition is S

In Android S, snapuserd binary was on vendor partition.
When there is an OTA update from S -> T, it is possible
that vendor partitions are not updated. In that case,
successive OTA updates T1 -> T2 will continue to have
snapuserd from Android S as vendor partition wasn't updated
to T. All this means, we should disable user-space snapshots.

When installing OTA during runtime, check for property
ro.vendor.build.version.release_or_codename; if the property
is set to "12", then skip userspace-snapshots.

Bug: 227614163

Test: Simulate OTA test on Pixel 6 from T1 -> T2 by forcefully
setting the property to 12 and verify OTA is applied
successfully by falling back to dm-snapshot.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I95f29145e5cd9ffb8d03d28ae414f0037b88be90