Revert "tabbox: Show window switcher only when there are two or more windows"

This reverts commit b31baaf0.

It's still a good idea to show the task switcher even if there's only
one window in order to provide the user feedback about their action.
Since the task switcher is not shown when there's only one window, it
can be confusing and lead to thinking that the task switcher is broken.
It also fixes a regression which prevents alt-tabbing to the only
remaining minimized window.

BUG: 480940
CCBUG: 419408


(cherry picked from commit 8086707d)