Separate stacks into Home and others.

Change-Id: I9434ee6cd9e8a8baf24d4e4ee5a15e99cf11f227
diff --git a/services/java/com/android/server/wm/DisplayContent.java b/services/java/com/android/server/wm/DisplayContent.java
index bcefc41..61aa0de 100644
--- a/services/java/com/android/server/wm/DisplayContent.java
+++ b/services/java/com/android/server/wm/DisplayContent.java
@@ -20,6 +20,8 @@
 import android.view.Display;
 import android.view.DisplayInfo;
 
+import static com.android.server.am.ActivityStackSupervisor.HOME_STACK_ID;
+
 import java.io.PrintWriter;
 import java.util.ArrayList;
 
@@ -193,6 +195,15 @@
         return false;
     }
 
+    void removeStackBox(StackBox box) {
+        final TaskStack stack = box.mStack;
+        if (stack != null && stack.mStackId == HOME_STACK_ID) {
+            // Never delete the home stack, even if it is empty.
+            return;
+        }
+        mStackBoxes.remove(box);
+    }
+
     /**
      * Reorder a StackBox within mStackBox. The StackBox to reorder is the one containing the
      * specified TaskStack.