libsnapshot: Export COW image size

Enable libsnapshot to export the size of the COW image, representing the
fraction of the COW device that is temporarily created in the /data
partition, computed just before initializing a merge operation.
Thise additional information can be used by other components (i.e.,
update engine) to enrich Virtual A/B metrics.

Bug: 154016862
Test: manual OTA
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: I4faa0785f23884e26161e0d51268dd7a305d86bf