layout: Reset the OSK to the primary monitor when monitors change

When monitors change, the previous index might not mean the same
physical monitor anymore, in fact, it might become invalid. In the
latter case, we'll actually get a JS error when accessing
this.keyboardMonitor in _updateKeyboardBox() . To avoid this, let's
just always reset the OSK to the primary monitor.

https://bugzilla.gnome.org/show_bug.cgi?id=738536