Ideally, if you have an AOSP build environment, you can build everything including Android SDK from scratch. However, it is much more convenient to have an Android SDK installation to help with virtual device creation or running emulator images.

You can always download the latest Android SDK from the following website:

https://developer.android.com/index.html

The host environment that we use in this book is Ubuntu 14.04. Download the Android SDK for Linux and decompress it to a folder in your Home directory.

The tools in Android SDK have been changed since API level 25. You may use an older version of Android SDK or the latest Android SDK so I gave the instructions for both cases here.