libsnapshot: Retrieve COW version from update engine manifest

update_metadata.proto will have the COW version. Retrieve
that from the manifest and compare it with the COW library.
If the versioning doesn't match, disable VABC.

The primary use case of this is during downgrade tests
in pre-submit. Whenever we have a COW format changes,
we may have to disable VABC for that specific transition
build. At a high level, the flow of version check will be:

1: Create a initial COW version of 1 in manifest (update_metadata.proto)
2: The latest COW version of libsnapshot is 2
3: libsnapshot will return VABC disabled
4: Check-in the CL and changes to manifest
5: Once the CL is baked in and the build is green, bump up the COW version to 2 in the manifest
6: Next set of tests, since both versions match, libsnapshot will enable VABC
7: Downgrade should be done to the build which was checked in at (5)

Bug: 183863613
Test: Apply OTA and verify if VABC is disabled if the versions don't
match
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Id55f33a90bb31b417e72f4fbe370daf05a68f05a