libsnapshot: Partially implement OpenSnapshotWriter.

Implement OpenSnapshotWriter for non-compressed Virtual A/B. This is
done by adding an OnlineKernelSnapshotWriter class, which forwards all
writes to a dm-snapshot block device.

This also introduces a new ISnapshotWriter class which extends
ICowWriter, and adds features specific to libsnapshot (versus ICowWriter
which is intended only for the new COW format). The OpenSnapshotReader
call has been moved here since the writer retains all the information
needed to create the reader.

To test the new call, vts_libsnapshot_test has been modified to use
OpenSnapshotWriter.

As part of this change, all consumers of libsnapshot must now link to
libsnapshot_cow.

Bug: 168554689
Test: vts_libsnapshot_test
Change-Id: Ieedfadc557833c1e0540922aabc6e95c80266a64