Upgrade bazlets to latest stable-2.15 to build with 2.15.7 API
Upgrade bazlets to latest stable-2.15 to build with 2.15.7 API

Change-Id: I8e204d846dd597a679d51d8dac7d7baba5418409

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