I have been struggling, struggling, struggling, trying all other proposed solutions. Didn't work. But what did is...

I normally put my sketches in a folder that is not the standard location. Furthermore it is not on the computer's 'home' disk.

I found that when I put the sketches into Processing's expected home directory and reassigning the path in preferences (and without doing any additional installs – I've done everything as said before) the sketch loaded to my external device properly, and also for the first time booted up an emulator (it was never creating one before). 

I then reassigned the path to my preferred processing sketch directory on the external drive and it worked building from there too. 

So. Processing has an issue with not updating a path somewhere when Android is installed 

If you do not use the standard sketch path, you are screwed as something is missing when Processing tries to do the build.

But resetting the path to your preferred sketch folder location after the Android installs fixes this. 

Whether it stays fixed is another matter.