Avoid per task app icons in Taskbar for desktop mode

Until Taskbar multi instance support was in place, Taskbar would show
multiple icons for different instances of the same app in desktop mode.
Now this is no longer needed as taskbar has multi instance support, so
now there is only one icon per instance of an app of the same user.

Fix: 382589460
Test: Started several Chrome windows -> Taskbar has 1 icon for chrome
Flag: com.android.launcher3.enable_multi_instance_menu_taskbar
Change-Id: I2df4327dcdc9d221164055ebc33ff508273e7ef9