snapuserd: Add an IBlockServerFactory abstraction.

To avoid SnapshotHandler hardcoding specifics about dm-user, this patch
adds a factory interface, responsible for providing IBlockServerOpener
objects.

The test harness will use this to facilitate dm-user-less testing on
host devices.

Bug: 288273605
Test: snapuserd_test
Change-Id: Ifd33c28ee7076f30a8a90f745353893188f97a08