Only show Desktop Wallpaper for the default display.

We should show the Desktop Wallpaper only if there is a freeform window
on the default display, as this is where the wallpaper will be shown
currently.

Bug: 352316177
Flag: EXEMPT bug fix
Test: atest DesktopTasksControllerTest
Change-Id: I04d5be168fef3bdf6e35fdf13af0499f9277af04