Use task's displayId in moveToBackground flows

Moving a background task to desktop involves "activating" the desk it'll
end up in, meaning, bringing up the home/wallpaper in front of other
non-desktop tasks, bringing up inactive tasks that already existed in
that desk and applying task-limit checks.

This function previously used DEFAULT_DISPLAY, so this CL changes that
to use the displayId available in the TaskInfo of the task.

Flag: com.android.window.flags.enable_desktop_windowing_mode
Flag: com.android.window.flags.enable_move_to_next_display_shortcut
Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 342378842
Test: m WMShellUnitTests
Change-Id: Ia7db80d6608f2ccb9807d8695a19d14fb54ea4c0