Avatica - Regenerate protobuf files for 3.6.1

I was looking into deprecation warnings and found that protobuf 3.6.0 should have added suppresswarnings to a few valueOf calls that were deprecated. However, the java files were not being regenerated even with the protobuf-maven-plugin.

It seems like the checkStaleness flag was preventing the compilation even after updating protobuf from 3.5.x to 3.6.1.

It doesn't look like there is a good reason to have checkStaleness set to true since the protobuf compilation is pretty quick. This should help us catch protobuf errors sooner.