User domain layer.

UserSwitcherActivity refactor: CL 5/7

This CL adds the domain layer, which is just the UserInteractor really.

Bug: 243844359
Test: Unit tests included. Also verified manually as part of the
testing of CL 7/7 in the chain.

Change-Id: I83e45a0060ab746874107c79bc8a1e2aeb07e409