Fix incremental build for autoFVT tasks

[fritze2]

• Current behavior is that the Gradle autoFVT task runs every time that a user runs buildfat for a FAT bucket. The reason it does is because we clean the directory where autoFVT outputs into on every run, whether there was a change or not, and we do not define inputs or outputs for the ad-hoc task.
• Expected behavior is the build shouldn't clean the autoFVTDir unless there was a change in the inputs, and the autoFVT task needs inputs and outputs defined correctly to determine when to run the clean.
• Saves time in re-builds locally, and could save time in remote builds if we restore caches of ~/.gradle and FAT's build/ directories.

[LibbyBot]

Code analysis and actions

DO NOT DELETE THIS COMMENT.

• Please describe in a separate comment how you tested your changes.