Merge branch 'stable-3.1'

* stable-3.1:
  Upgrade bazlets to latest stable-2.16 to build with 2.16.18 API
  Bump Bazel version to 3.1.0
  Bump Bazel version to 3.0.0

Change-Id: I45061ed0d913d6f33c8dcfed6a3b83b7efd57c3b