獲取棧頂的activity

	/**
	 * 獲取棧頂的activity
	 * @param context
	 * @return
	 */
	public static String getTopActivity(Context context) {
		ActivityManager manager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
		List<RunningTaskInfo> runningTaskInfos = manager.getRunningTasks(1);

		if (runningTaskInfos != null)
			return runningTaskInfos.get(0).topActivity.getClassName();
		else
			return "";
	}