Xiaomi SDK works

[weihengSu2]

Things to consider before you submit the PR:

• [x] Are tests passing locally?
• [x] Are the files formatted correctly?
• [ ] Did you add unit tests?
• [ ] Did you test your change with either the sample apps that are included in the repository or with a blank app that uses your change?

Description

The PR will contain the works in https://github.com/Azure/azure-notificationhubs-dotnet/pull/261 https://github.com/Azure/azure-notificationhubs-dotnet/pull/262 https://github.com/Azure/azure-notificationhubs-dotnet/pull/263

[marstr]

We're going to keep this work in a separate feature branch for the time being. Closing this PR for now.

[marstr]

When the time comes, we'll either re-open this PR or create a new more or less identical one.