What is the best sutiable way to keep my user's data in Android

I want to add user authorize system in my Android application. At first glance it looks very easy, however; when I start to research, I encountered too many problems. Because every person can download my apk file and can see my auth system code and can send HTTP POST requests from anywhere.

Firstly I founded that link below

http://ift.tt/1kK5Anb

It was looked me very complex and I thought that there can be more simple ways that can be fee.

I tried to use Amazon Cognito and completed all login protocols using facebook and google+. Finally I encountered a big frustrating problem. If user would use different login system in different devices, Cognito can not understand that are the same user. I asked this problem in Cognito forums and their offical answer is below.

We use tokens issued by providers to determine identity. If you login using Facebook on one device and you use Google on another device, there is no way for us to determine that these tokens belong to the same user

then I decided to use Google Cloud mobile Solutions but I learned that It is banned in China mainland which My app will also be Chinese.

parse.com is very slightly but It does not have Google plus login.

I haven't only tried Microsoft Azure because Its documentations are very confusing. But If I can be sure that It compeletely works, I would learn everything about Azure.

I am enhuaist to search the best suitable way save my users data on remote server. What is your suggestions ? Which way should I follow ?

Thank you.