Don't try to sync wallpapers

Wallpapers currently don't support sync, so it doesn't make sense
to try and sync with them -- otherwise we get timeouts waiting for
a signal that will never be fired

Bug: 238328090
Test: atest NexusLauncherTests (and observe logs for no related timeouts)
Change-Id: I83a3489c0790975eb80a7030cf3d1129d4746513

services/core/java/com/android/server/wm/WindowState.java

@@ -5927,6 +5927,10 @@ class WindowState extends WindowContainer<WindowState> implements WindowManagerP
         if (!super.prepareSync()) {
             return false;
         }
+        if (mIsWallpaper) {
+            // TODO(b/233286785): Add sync support to wallpaper.
+            return false;
+        }
         // In the WindowContainer implementation we immediately mark ready
         // since a generic WindowContainer only needs to wait for its
         // children to finish and is immediately ready from its own