Filter out DesktopWallpaperActivity task from TTT.

Having DesktopWallpaperActivity in TTT sometimes cause crashes as it's a
system activity and launcher doesn't have the permission to read its
package info.

Bug: 398943577
Test: manually verify that WallpaperActivity is filtered out
Flag: EXEMPT Bug fix
Change-Id: I305763f5393350127e4046f1fcf7f3d8ed883f25