Begin switch over to task based history.

- Introduce the task history and add to and remove from it with
verification.

Change-Id: If97e74f5a13f85acdb1521fc6d0b066a7e8584ae
diff --git a/services/java/com/android/server/wm/TaskList.java b/services/java/com/android/server/wm/TaskList.java
index 88791f2..67dfa4f 100644
--- a/services/java/com/android/server/wm/TaskList.java
+++ b/services/java/com/android/server/wm/TaskList.java
@@ -27,4 +27,9 @@
         mAppTokens.add(wtoken);
         mDisplayContent = displayContent;
     }
+
+    @Override
+    public String toString() {
+        return "id=" + taskId + " appTokens=" + mAppTokens;
+    }
 }