Fix bug where switching users didn't dismiss UI.

There's a bug where, with the new implementation, when the user switches
to a different user on a phone, the user switcher dialog is not properly
dismissed.

This CL takes care of that by properly dismissing the DialogShower, if
one is passed in.

Fix: 251733467
Test: Tests included. Verified that the bug no longer reproduces after
the fix.

Change-Id: I053eec01051a863e375906c17a64932aa865df7e