Merge branch 'stable-3.1' into stable-3.2

* stable-3.1:
  ContentTypeUtil: Add singleton scope
  Upgrade bazlets to latest stable-3.1 to build with 3.1.12 API
  Upgrade bazlets to latest stable-3.0 to build with 3.0.15 API
  Upgrade bazlets to latest stable-2.16 to build with 2.16.26 API
  Upgrade bazlets to latest stable-2.16 to build with 2.16.23 API

Change-Id: I64539e95e56258e182ff76e509cb8f7d22d5fd9d