Fix gradlew to allow offline builds

When running `./gradlew` the first thing it tries to do is download gradle:

Downloading https://services.gradle.org/distributions/gradle-5.2.1-all.zip

There seems to be no way to skip this, even if the correct version of gradle has already been downloaded and exists in the correct place. The tool should be smart enough to do a version check and skip downloading if it exists. Unfortunately, the logic is wrapped up inside gradle/wrapper/gradle-wrapper.jar so there does not seem to be any way to fix this. Where is the code for this jar?

This is the first step of several to allow beam to be built offline.