Remove build timestamp from BuildConfig

[friedger]

Use paperwork instead of BuildConfig for gitSha and timestamp

Paperwork is available at
https://github.com/zsoltk/paperwork

[tasomaniac]

I've asked about this in a Slack group. I think we should fix that but I am against adding additional plugin.

Instead of new Date() using gitTimestamp improves it a lot

def gitTimestamp = 'git log -n 1 --format=%at'.execute([], rootDir).text.trim()

It will do dexing when git timestamp changes which happens when we commit. It think it is ok to do that. Dexing is done every time we change java code anyways.