Add presubmit check for prebuilt jars

[thirtyseven]

Add presubmit check to ensure prebuilt jars are updated in the same PR where their sources change. This will prevent issues like this one, as well as:

• ensure that integration tests for changes to Java code run in the same PR where the changes are introduced
• allow end users to test these changes more easily by using git_override

[shs96c]

I love this. There's a handful of PRs that are currently in flight. It seems unfair to ask them to run more steps, so once those land, I'll merge this.