libsnapshot: Remove ISnapshotWriter.

This only added one real method to ICowWriter, so let's just fold it
into ICowWriter. CompressedSnapshotWriter goes away as well.
CompressedSnapshotReader is now part of libsnapshot_cow.

Bug: 280529365
Test: m otapackage
      apply ota
Change-Id: I55358c3c9be111d5aee1a0c22c29cb1539d05494