Need documentation on proper way to build from source

[ryao]

Right now, there is no documentation on how to build from source. After reading through Google's Android documentation and trial/error, I was able to build from source, but the Google Drive integration broke. It could be that there is something obvious only to people with Android development experience. Since I had none prior to writing a few patches that I sent, it was a pain.

The lack of documentation (in addition to the lack of a published license as I stated in #292) is not a great way to solicit third party contributors. I suggest writing something up so that a third party could do builds locally. That means documenting:

1. That you need to install gradle.
2. That you need Java 8.
3. That you need to install git and do a checkout (this much I knew, but I don't think instructions should assume such knowledge)
4. That you need to install the Android SDK updater. (I also knew this)
5. That you need the Android SDK build tools version 24.0.2.
6. All of the stuff in Extras that you need.
7. How to get a app/google-services.json file.
8. How to make the Google drive integration work with the app/google-services.json file.
9. How to do a build with gradle.
10. How to get it onto the Android TV. e.g. enabling debugging in developer mode, running adb connect $IP and doing adb install -r /path/to/apk/.
11. Where the apk can be found after it is built.