SurfaceFlinger: Ensure relatives are not detached with children.

A misapplication of traverseInZOrder...unfortunately we will have to watch
out for this. This code was also mistakenly accessing mDrawingState.

Test: Transaction_test.cpp
Change-Id: I6aa51885f52edff89f1ed5512408bf0a5a227af5
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index 80987a4..22a72f2 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -1661,17 +1661,14 @@
 }
 
 bool Layer::detachChildren() {
-    traverseInZOrder(LayerVector::StateSet::Drawing, [this](Layer* child) {
-        if (child == this) {
-            return;
-        }
-
+    for (const sp<Layer>& child : mCurrentChildren) {
         sp<Client> parentClient = mClientRef.promote();
         sp<Client> client(child->mClientRef.promote());
         if (client != nullptr && parentClient != client) {
-            client->detachLayer(child);
+            client->detachLayer(child.get());
+            child->detachChildren();
         }
-    });
+    }
 
     return true;
 }