User switcher - Update when multiple users enabled/disabled

Listen for the overall multiuser setting and update the available list
of users.

Also remove an unused preference.

Fixes: 228339026
Test: atest UserSwitcherControllerTest
Change-Id: Idbaf97c599bacba2122cbce2d047eb59262ac63e
Merged-In: Idbaf97c599bacba2122cbce2d047eb59262ac63e