Remove user switcher when multiple users is disabled

Removed weird behavior that directed the user to a contact if multiple
users was disabled.

The code in MultiUserSwitch closely follows that of UserManager in order
to short-circuit things properly.

Test: manual on QS and Keyguard status bar
Bug: 115645159
Change-Id: I8f1656c8ef77d486723a95ec4cfaa145a449bef6