Fix the condition for removing desktop wallpaper

This change fixes a condition that led to an ANR error when a task was
moved from the default display to another.

Previously, DesktopWallpaperActivity was removed when the last desktop
task was removed (e.g. the task closed or left desktop mode). However,
if other desktop tasks remained on a different display,
DesktopWallpaperActivity remained on the default display. In this case,
DesktopWallpaperActivity became the topmost task on the default display,
and the system server attempted to give it focus. Because
DesktopWallpaperActivity is not focusable, the system server could not
find a focusable window, eventually causing an ANR.

This commit corrects the condition for removing the desktop wallpaper.
Now, DesktopWallpaperActivity is removed when the last desktop task on
the default display is removed.

Bug: 381361395
Test: DesktopTasksControllerTest#moveToNextDisplay_removeWallpaper
Flag: com.android.window.flags.enable_per_display_desktop_wallpaper_activity
Change-Id: I7f142b3cc4282109c716e80b65f8b9e2cc343787