Revert "Revert "1/ Create display content window controller to position stacks in the display""

This reverts commit 7ef6d3371842748eebc3361eedc2dd492b39cfad.

Reason for revert: Testing relanding changes with ag/3515280

Change-Id: I44c9dcd3c68f05d57ffb47c63c08a30279bb5b22
diff --git a/services/core/java/com/android/server/wm/DisplayContent.java b/services/core/java/com/android/server/wm/DisplayContent.java
index 6dc384a..d7a58d9 100644
--- a/services/core/java/com/android/server/wm/DisplayContent.java
+++ b/services/core/java/com/android/server/wm/DisplayContent.java
@@ -1802,6 +1802,11 @@
         getParent().positionChildAt(position, this, includingParents);
     }
 
+    void positionStackAt(int position, TaskStack child) {
+        mTaskStackContainers.positionChildAt(position, child, false /* includingParents */);
+        layoutAndAssignWindowLayersIfNeeded();
+    }
+
     int taskIdFromPoint(int x, int y) {
         for (int stackNdx = mTaskStackContainers.getChildCount() - 1; stackNdx >= 0; --stackNdx) {
             final TaskStack stack = mTaskStackContainers.getChildAt(stackNdx);