Show a dialog when user switching fails

When user switch fails an alert dialog will pop up letting the user know
the switching failed.

If the switch failed because switching is not allowed, the dialog will
have a button that leads to User Settings.

Bug: b/376916536
Test: manual
Flag: android.multiuser.multiuser_widget
Change-Id: I74ed00df4a2275cb738dd6d1c7de01d6f44480ed