Complete removal of Task from DisplayContent.

Moved Tasks into TaskStacks.

Change-Id: I3478dab4eab3a68d4d71a7a5bb2e65ba2394f2d7
diff --git a/services/java/com/android/server/wm/WindowState.java b/services/java/com/android/server/wm/WindowState.java
index ca6d2ca..e471e46 100644
--- a/services/java/com/android/server/wm/WindowState.java
+++ b/services/java/com/android/server/wm/WindowState.java
@@ -685,6 +685,13 @@
         return mDisplayContent.getDisplayId();
     }
 
+    TaskStack getStack() {
+        if (mAppToken != null) {
+            return mService.mTaskIdToTask.get(mAppToken.groupId).mStack;
+        }
+        return null;
+    }
+
     public long getInputDispatchingTimeoutNanos() {
         return mAppToken != null
                 ? mAppToken.inputDispatchingTimeoutNanos