Merge branch 'stable-3.1'

* stable-3.1:
  Update bazlets to latest stable-3.1 to build with 3.1.4 API
  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: I49c51da68611dc83da49be8fd9a95105b5d688f4