Exclude WindowState from transition targets

Otherwise it may look weird to check whether the WindowState can
promote to ActivityRecord. The case happens when transferring
splash screen window, which will collect the starting window.
And usually the window level animation is done by someone other
than shell.

Bug: 194112093
Test: atest SurfaceControlTest
Change-Id: I11868342b9a76a0561718f0e549b412eb4282f6a

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

@@ -1039,6 +1039,8 @@ class Transition extends Binder implements BLASTSyncEngine.TransactionReadyListe
                         "  Rejecting as detached: %s", wc);
                 continue;
             }
+            // The level of transition target should be at least window token.
+            if (wc.asWindowState() != null) continue;
 
             final ChangeInfo changeInfo = changes.get(wc);