Upgrade bazlets to latest stable-2.15 to build with 2.15.9 API
Upgrade bazlets to latest stable-2.15 to build with 2.15.9 API

Change-Id: I8a7318f1b5376c6f7b11dc4e784a4be07aeec234

GerritCodeReview/plugins_admin-console,GerritCodeReview/plugins_admin-console,GerritCodeReview/plugins_admin-console