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/tests/servicestests/src/com/android/server/am/ActivityTestsBase.java b/services/tests/servicestests/src/com/android/server/am/ActivityTestsBase.java
index 96bf49b..10253c5 100644
--- a/services/tests/servicestests/src/com/android/server/am/ActivityTestsBase.java
+++ b/services/tests/servicestests/src/com/android/server/am/ActivityTestsBase.java
@@ -30,6 +30,7 @@
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.spy;
 
+import com.android.server.wm.DisplayWindowController;
 import org.mockito.invocation.InvocationOnMock;
 
 import android.app.IApplicationThread;
@@ -345,7 +346,7 @@
         }
     }
 
-    private static class TestActivityDisplay extends ActivityDisplay {
+    protected static class TestActivityDisplay extends ActivityDisplay {
 
         private final ActivityStackSupervisor mSupervisor;
         TestActivityDisplay(ActivityStackSupervisor supervisor, int displayId) {
@@ -374,6 +375,11 @@
                         this, stackId, mSupervisor, windowingMode, activityType, onTop);
             }
         }
+
+        @Override
+        protected DisplayWindowController createWindowContainerController() {
+            return mock(DisplayWindowController.class);
+        }
     }
 
     private static WindowManagerService prepareMockWindowManager() {