[User Switcher] dismiss dialog appropriately.

Dismisses the dialog in the phone user switcher when an action is
selected. This will dismiss the dialog when adding a user, adding a
supervised user, and going to manage users settings.

Bug: 254704051
Test: Unit tests and open phone user switcher. I spammed the buttons,
and observed that the dialog has disappeared. I switched users and
created guests.

Change-Id: Ib3929c8b7d95c5aed6da0c18c10a2c1f5793d163