user UI layer changes to support USC dep removal

Minor changes to the view-model class and the addition of a
UserSwitcherDialogCoordinator to be able to show and dismiss dialogs
based on state exposed by the interactor.

Bug: 246631653
Test: unit tests for new coordinator class included, manually verified
the operations of the full-screen user switcher, the smaller dialog, the
footer in quick settings, and the dropdown switcher on the lock-screen
bouncer.

Change-Id: I058541bb5c31023735121bbad54b93afba73a89a