snapuserd: Rename libsnapshot_snapuserd to libsnapuserd_client.

We don't need this on the host, since the host will not be running
snapuserd as a server. Rename it for clarity and remove it where we can.

Bug: 288273605
Test: snapuserd_test
Change-Id: I679ef668a89411c670fea8d3b758bde589623548