Enable checker framework nullness checker

[anuraaga]

I've always been interested in using checker framework in builds but never tried since I remember the configuration being tedious (setup a custom JDK, etc). But looking at the docs for the first time in a while, I don't see much tedium anymore

https://github.com/kelloggm/checkerframework-gradle-plugin

Might be nice to give it a look. We keep an eye out for nullable, so I'd expect there not to be too many warnings when adding it, but if there are that's also a good thing to find them :)

[trustin]

Related: #1680