簡單來說：
getDeviceToken 方法有三：
一、ionic Native Push (https://ionicframework.com/docs/v2/native/push/)
二、ionic Native Firebase (https://ionicframework.com/docs/v2/native/firebase/)
三、cordova FCM plugin (https://github.com/fechanique/cordova-plugin-fcm)
之前試用的方式是：
http://sueboy.blogspot.tw/2017/01/ionic2-fcm-send-message-to-device-token.html
．AngularFire2 authstate.auth.getToken() is JWT token for firebase
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