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: Ic671c91c32e9bf24695b067ff97cf9fa65534b1e
Merged-In: Ic671c91c32e9bf24695b067ff97cf9fa65534b1e