关联 mServiceHandler
mServiceHandlermsg.replyTo中
iBinder实例（mServiceMessenger）
onBind()中返回mServiceMessenger.getBinder()
mClientMessenger
msg.replyTo = mClientMessenger
服务器
客户端
mClientHandler
拿到mClientMessenger
关联 mClientHandler
使用mServiceMessenger发送msg到服务器的mServiceHandler中
在onServiceConnected中通过new Messenger(iBinder)
mServiceMessenger
拿到mServiceMessenger
mClientMessenger.send(Message msg)
mServiceMessenger.send(Message msg)
使用mClientMessenger发送msg到客户端的mClientHandler中
