Only allow activities to move to full screen stack.

We allowed activities to move to any stack, but that's too much
freedom. Instead we only allow them to move from freeform stack to a
fullscreen stack.

Change-Id: I04de9bbf18cf4431d7bd34d6c727de82802661ef
diff --git a/core/java/com/android/internal/widget/DecorCaptionView.java b/core/java/com/android/internal/widget/DecorCaptionView.java
index c3fe9e7..409a17f 100644
--- a/core/java/com/android/internal/widget/DecorCaptionView.java
+++ b/core/java/com/android/internal/widget/DecorCaptionView.java
@@ -16,8 +16,6 @@
 
 package com.android.internal.widget;
 
-import static android.app.ActivityManager.StackId.FULLSCREEN_WORKSPACE_STACK_ID;
-
 import android.content.Context;
 import android.graphics.Color;
 import android.graphics.Rect;
@@ -351,7 +349,7 @@
         Window.WindowControllerCallback callback = mOwner.getWindowControllerCallback();
         if (callback != null) {
             try {
-                callback.changeWindowStack(FULLSCREEN_WORKSPACE_STACK_ID);
+                callback.exitFreeformMode();
             } catch (RemoteException ex) {
                 Log.e(TAG, "Cannot change task workspace.");
             }