PendingIntent是一个特殊的Intent,实际上它像一个邮包，其中包裹着真正的Intent，当邮包未打开时，Intent是被“挂起”的，所以并不执行，
只有当邮包拆开时才会执行。它与Intent的区别在于：Intent 是及时启动，intent 随所在的activity 消失而消失。
PendingIntent 可以看作是对intent的包装，通过getActivity,getBroadcast ,getService来得到pendingintent的实例，当前activity并不能马
上启动它所包含的 Intent, 而是在外部执行 pendingintent时再调用 Intent。
正由于pendingintent中 保存有当前App的Context，使它赋予外部App一种能力，使得外部App可以如同当前App一样的执行pendingintent里
的 Intent， 就算在执行时当前App已经不存在了，也能通过存在pendingintent里的Context照样执行Intent。常和alermanger 和notificationmanager
一起使用。
Intent一般是用作Activity、Sercvice、BroadcastReceiver之间传递数据，而Pendingintent，一般用在 Notification上，可以理解为延迟执行的
intent，PendingIntent是对Intent一个包装。
FLAG_ONE_SHOT:this PendingIntent can only be used once. If set, after send() is called on it, it will be automatically canceled
for you and any future attempt to send through it will fail.
FLAG_NO_CREATE:if the described PendingIntent does not already exist, then simply return null instead of creating it.
FLAG_CANCEL_CURRENT:if the described PendingIntent already exists, the current one is canceled before generating a new one.
You can use this to retrieve a new PendingIntent when you are only changing the extra data in the Intent; by canceling the previous
pending intent, this ensures that only entities given the new data will be able to launch it. If this assurance is not an issue, consider
FLAG_UPDATE_CURRENT.
FLAG_UPDATE_CURRENT: if the described PendingIntent already exists, then keep it but its replace its extra data with what is in
this new Intent. This can be used if you are creating intents where only the extras change, and don't care that any entities that received
your previous PendingIntent will be able to launch it with your new extras even if they are not explicitly given to it.
上面4个flag中最经常使用的是FLAG_UPDATE_CURRENT，因为描述的Intent有更新的时候需要用到这个flag去更新你的描述，否则组件在下次事件
发生或时间到达的时候extras永远是第一次Intent的extras。
使用FLAG_CANCEL_CURRENT也能做到更新extras，只不过是先把前面的extras清除，另外FLAG_CANCEL_CURRENT和
FLAG_UPDATE_CURRENT的区别在于是否创建一个新的Intent，FLAG_UPDATE_CURRENT能够创建一个新的Intent，而FLAG_CANCEL_CURRENT
则不能，只能使用第一次的Intent。
另外两flag就比较少用，利用FLAG_ONE_SHOT获取的PendingIntent只能使用一次。利用FLAG_NO_CREAT获取的 PendingIntent若描述的
Intent不存在则返回NULL值.