Configure plugin repositories to use Gradle Portal last

Summary:
Gradle Plugin Portal proxies jcenter which is quite unstable these days. This change updates plugin repositories to look into maven central and google first and use gradle plugin repo only as a fallback.

Changelog: [Internal] - Prioritize maven central for Gradle plugins

Reviewed By: cortinico

Differential Revision: D33550827

fbshipit-source-id: b436b05b0fd07865b56dd3e442d8399678dfff85