android-bitmap的缓存策略
public linkedhashmap(int initialcapacity, float loadfactor, boolean accessorder) {super(initialcapacity, loadfactor);init();this.accessorder = accessorder;}所以在lrucache中应该选择accessorder = true，当我们调用put、get方法时，linkedhashmap内部会将这个item移动到链表的尾部，即在链表尾部是最近刚刚
