Remove user switcher callback when exiting user switcher mode

It was previously not possible to exit user switcher mode in the
bouncer. Now with SIM support on large screens, it is.

Fixes: 369996464
Test: atest KeyguardSecurityContainerTest
Flag: EXEMPT bugfix
Change-Id: I9bcd24d15b823591c11b2c568311f0a3a6478eec