snapuserd: Add a harness to run tests without dm-user specific code.

This patch adds an abstraction layer around Tempdevice (which wraps
device-mapper), and a layer to replace hardcoding of DmUserBlockServer.

The only implementation of the new layer, currently, is for dm-user.
However this will allow the harness to run with a backend chosen at
runtime, making testing on the host or of ublk much easier.

Bug: 288273605
Test: snapuserd_test
Change-Id: I8735ef6c373f3e5c5cdf3df461668ddd8e551f63