Revert^2 "If wallpaper activity already exists, reorder it"

This reverts commit 7db40104.

Reason for revert: Previous test failure should be fixed with ag/31508353

Bug: 391501236
Test: Manual & atest DesktopTasksControllerTest
Flag: com.android.window.flags.enable_desktop_wallpaper_activity_for_system_user

Change-Id: I2fe2f44b960a7a0187a0d1cbca995f415c7bdac4