Prepares UserSwitcherController.

UserSwitcherActivity refactor: CL 1/7

- Extracts some logic out of UserSwitcherController and into LegacyUserUiHelper. This logic will later be used by classes in the modern architecture refactor of UserSwitcherActivity. The code is being moved to a common place to avoid code duplication.
- Exposes some new APIs in UserSwitcherController to facilitate user
  selection, expose guest-specific and lock-screen specific
  configuration, and trigger dialog-based user journeys such as adding a
  user

Bug: 243844359
Test: Existing unit tests still pass. Manually verified in CL 7/7.
Change-Id: Iab77acb20781b291289a59389db7b542fdfe0a47