Disable split screen option in app menu in Desktop Mode

The split screen app shortcut is not intended to work in Desktop Mode.
This change removes it from Taskbar and all apps in this mode to avoid
a crash.

Fix: 346333758
Test: Launch the app menu for different apps in Desktop mode and in
normal mode. Check that the split option only shows up in normal mode.
Flag: EXEMPT bugfix

Change-Id: I6e2535b41a490ad3eb5c0ebd924c49eaf5028669