When closing task stack remove reference to display content.

This tightens the guarantee that detached stack won't be used. We also
add logging to detecting a situation where a stack not belonging to a
display is being moved on that display.

Bug: 22191609
Change-Id: Ia674bb5960018104a56c5138775ab5216906675b

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

@@ -203,7 +203,9 @@ class DisplayContent {
     }
 
     void moveStack(TaskStack stack, boolean toTop) {
-        mStacks.remove(stack);
+        if (!mStacks.remove(stack)) {
+            Slog.wtf(TAG, "moving stack that was not added: " + stack, new Throwable());
+        }
         mStacks.add(toTop ? mStacks.size() : 0, stack);
     }
 

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

@@ -385,8 +385,6 @@ public class TaskStack {
         }
 
         close();
-
-        mDisplayContent = null;
     }
 
     void resetAnimationBackgroundAnimator() {
@@ -518,6 +516,7 @@ public class TaskStack {
             mDimLayer.destroySurface();
             mDimLayer = null;
         }
+        mDisplayContent = null;
     }
 
     public void dump(String prefix, PrintWriter pw) {