Cleans up UserSwitcherController API.

* Removes unused method and classes
* Makes externally-unused methods non-public

Test: Built and ran System UI, manually interacted with user switcher UI
through the lock-screen and full-screen experiences
Bug: 246631653

Change-Id: I0d6a8e4ffc899f358533486d8c4b0d38de6e5519