liblp: Add an attribute to indicate that a partition has been updated.

When the bootloader (or fastbootd) flashes the super partition, we need
to make sure that init doesn't re-map any snapshot or snapshot-merge targets.
A simple way to do this is to introduce an attribute that is only added
by update_engine. When this flag is present, we know the partition has
not been flashed.

This bumps the minor version of LpMetadata. To make this as uninvasive
as possible, the new minor version is only used when MetadataBuilder
detects the new attribute. The new liblp can read older metadata, but will
reject it if it contains an illegal attribute set.

Bug: 139154795
Test: liblp_test gtest
Change-Id: I5ae15d11219b41575a9f71d7dbdb43cbf07a3529