What is the "Android Sooner Single ADB Interface"? What is the "Android Sooner Single ADB Interface", what function does it serve, and why does it have that name?  Specifically, to what does the term "Sooner" refer?

It appears to be a driver written by Google that gets installed on desktop computers when some Android devices are attached, but the name is so odd that it's worthy of having a canonical answer providing a full explanation as to its function and name.
The model names of released mobile devices are created by marketing departments not that long before the release. Before that moment engineers use unofficial code names to refer to the products they are working on. 

The Sooner was the code name of the 1st ever Android prototype device built by HTC for Google back in 2007.