User UI layer.

UserSwitcherActivity refactor: CL 6/7

Includes view-models and view-binders for the user and user-switcher
experiences.

Bug: 243844359
Test: Integration tests included for the top-most view-model. Also
manually tested with the next CL.

Change-Id: Ia05c2f267a37475c409ffed9b9784cdc435a9d76