libsnapshot:snapuserd: Transitions for snapuserd

1: Create a static library which exposes APIs
to manage snapuserd daemon.

2: Snapuserd daemon creates communication socket.

Bug: 168258493
Test: cow_snapuserd_test tests all the library API
along with the IO path from dm-snapshot.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I8aaedf89d75e793c145fdad248a4d88e0ce8348c