Introduce concept of DockUser

Dreams and docking often refers to "user zero" or "system user".
But more generally, the special user for dreams or switching-to-on-docking
need not be user 0, and in fact won't be in certain modes. So this needs updating.

Test: the files mentioned in the topic
Bug: 257333623
Change-Id: If068aa79a770bed4abf3f7ca1859a95cd1eb4ecd