Removed WallpaperController dependency on WindowList.

WallpaperController now accesses the container hierarchy directly
to determine the state of the wallpaper windows and targets.

Bug: 30060889
Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test \
      android.server.cts.ActivityManagerTransitionSelectionTests

Change-Id: Ib70beaf340f257ad4e1093cc127f81e7adf41636