Activate DesktopWallpaper when fullscreen task is forced to be freeform

This CL lets the controller launch the desktop wallpaper when an opening
fullscreen task is forced to be freeform.
Otherwise, we can unexpectedly see the Launcher activity even after the
desktop task is opened.

Bug: 375370653
Bug: 375375114
Flag: EXEMPT bug fix
Test: DesktopTasksControllerTest
Change-Id: I3df6ef1d0ca8d7e1c796adfac52d8e4bab938bec