Freeform to recents app transition.

Bug: 24913782

Change-Id: I54fcbe38c51e5d75fa5ad2cb38de89d371b47bed
diff --git a/services/core/java/com/android/server/am/ActivityStack.java b/services/core/java/com/android/server/am/ActivityStack.java
index 1cd3758..51042cb 100644
--- a/services/core/java/com/android/server/am/ActivityStack.java
+++ b/services/core/java/com/android/server/am/ActivityStack.java
@@ -4376,7 +4376,7 @@
 
             RunningTaskInfo ci = new RunningTaskInfo();
             ci.id = task.taskId;
-            ci.stackId = (task.stack == null) ? INVALID_STACK_ID : task.stack.getStackId();
+            ci.stackId = mStackId;
             ci.baseActivity = r.intent.getComponent();
             ci.topActivity = top.intent.getComponent();
             ci.lastActiveTime = task.lastActiveTime;