Keyboard layout indicator on mate-panel does not update if user sets "while pressed" shortcut

[N0rbert]

Steps to reproduce:

1. Install Ubuntu 16.04 LTS MATE (v. 1.12.1) or 18.04 LTS MATE (v. 1.20.1)
2. Add two keyboard layouts (for example, English and Russian)
3. Open mate-keyboard-properties
   3.1. Click Options
   3.2. In opened window expand Switching to another layout
   4.4. Check checkbox named Right Alt (while pressed) (for example).
   4.5. Close GNOME Tweaks windows
4. Test layout switching - press and write some text

Expected results:

• keyboard layout indicator shows second keyboard layout when user holds button and shows the first when it is not pressed

Actual results:

• keyboard layout indicator shows only the first keyboard layout

This bug is known on Ubuntu as bug 1779333