Merge branch 'stable-3.3'
* stable-3.3:
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-3.2 to build with 3.2.6 API
Upgrade bazlets to latest stable-3.1 to build with 3.1.11 API
Upgrade bazlets to latest stable-2.16 to build with 2.16.23 API
Change-Id: Ifed03352924be4a62978f2672d3c50d5ad9e0505