Yao Wei authored Aug 29, 2022 and Yao Wei committed Aug 31, 2022

Due to !2056 the monitor config is 2 instead of 4 on the desktop.
However, the constant `Meta.MonitorSwitchConfigType.UNKNOWN` is fixed to
4, and may cause the initial selection to be out of index thus fail to
display the switcher.  This replaces the above constant to the actual
length of the switcher items.

Part-of: <GNOME/gnome-shell!2451>