Summary:
Update some build dependencies (gradle, android plugin) and make some
minor cleanups in the gradle files along the way.
Pull Request resolved: https://github.com/facebook/Sonar/pull/151
Reviewed By: priteshrnandgaonkar
Differential Revision: D8818841
Pulled By: passy
fbshipit-source-id: 78b6592dbf967553aadead57a60023a0b5ae79b6