Don't allow users to be switched on keyguard when multiple users is disabled

Currently, if there is only one user, and "add users from lockscreen" is  disabled, the Keyguard user-switcher dropdown sticks around to display the current user.

The changes here will maintain visual parity with that user experience when "multiple users" is disabled.

Bug: b/269399217
Test: atest BaseUserSwitcherAdapterTest
Change-Id: Id808e3ecd34de57f19976101eb1a9c37831f26b9