Do not reposition leaf tasks for single task stacks

Reposition leaf tasks may cause wrong top focused stack while switch
user.

Bug: 183996163
Test: atest WmTests:TaskTests
Change-Id: Ifee5b2092e73b26bc0b82afe32a9c803d0d88917
diff --git a/services/core/java/com/android/server/wm/Task.java b/services/core/java/com/android/server/wm/Task.java
index a4a0866..549be43 100644
--- a/services/core/java/com/android/server/wm/Task.java
+++ b/services/core/java/com/android/server/wm/Task.java
@@ -5534,7 +5534,7 @@
         mCurrentUser = userId;
 
         super.switchUser(userId);
-        if (isLeafTask() && showToCurrentUser()) {
+        if (!isRootTask() && showToCurrentUser()) {
             getParent().positionChildAt(POSITION_TOP, this, false /*includeParents*/);
         }
     }