Merge branch 'stable-3.0' into stable-3.1

* stable-3.0:
  Upgrade bazlets to latest stable-2.16 to build with 2.16.18 API
  Bump Bazel version to 3.1.0

Change-Id: I0c962e8628e6a93b0d9b72d55c9e77ebaa57b2fc