Synced with https://github.com/googlesamples/android-play-billing/commit/7a94c6905a9c125518354c216b5c3094fde47ce1.
It had been synced with style changes (spaces -> tabs), not sure why I accepted to merge it this way back then... Synced with https://github.com/google/play-licensing/commit/eb57657f666363914085cdde49d875cf49f5ab06, same as before. Custom-changes will be reapplied in the next commit, if relevant.
-Added new Gradle build system, as it is the required build system