Gate new desktop task menu button with device config check

Desktop windowing is enabled only when the desktop windowing flag is
enabled and the device is supported through setting a device config
flag. Add this device config check to the desktop task menu button so it
does not appear if the device is not compatible.

Flag: NONE
Fixes: 335178869
Test: atest -c NexusLauncherTests:com.android.quickstep.DesktopSystemShortcutTest
Change-Id: Ia2559c1ff57bb7921867e69a47d868c03eae57e7