Generate a root for wallpaper-only transitions

Putting secondary display wallpapers under the primary display's
transition root during a switch of wallpaper visibility is wrong.
If all of the windows on a display are wallpapers, there should be
no problem creating a transition root for them at the LCA like with
Tasks.

Flag: EXEMPT bugfix
Test: atest com.android.server.wm.TransitionTests
Bug: 364879728
Change-Id: I4f7e7bfa69b1268e1a8275e70161792dfa8bb0e2