Fix PinnedStackTests#testPinnedStackWithDockedStack

The split-screen-primary root task's windowing mode was
reset while moving a split-screen-primary task to
split-screen-primary windowing mode.

Avoid doing so if the task is already in split-screen-primary.

Bug: 153843333
Test: atest PinnedStackTests
Change-Id: I058ee89c42adc0ed0902c565ebda87fc60843179

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

@@ -2786,6 +2786,11 @@ public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
         }
 
         final int prevMode = task.getWindowingMode();
+        if (prevMode == windowingMode) {
+            // The task is already in split-screen and with correct windowing mode.
+            return true;
+        }
+
         moveTaskToSplitScreenPrimaryTask(task, toTop);
         return prevMode != task.getWindowingMode();
     }