User data layer.

UserSwitcherActivity refactor: CL 4/7

This is the data layer for the UserSwitcherActivity refactor. It
includes a repository and a couple of shared models that the repository
exposes.

Bug: 243844359
Test: Unit tests included. Also tested as part of the big manual test in
CL 7/7 in this chain

Change-Id: I4cb5b1c4f018def88cda5864de51c644c6d8ce0f