Add react build app plugin and extension

Summary:
Changelog:
[Android][Added] - Basic definition for react gradle plugin

Adds plugin and build configuration + copies config from react.gradle to extension.
Copies internals of react.gradle to the plugin. Will be refactored in the next commits.

Reviewed By: mdvacca

Differential Revision: D25693008

fbshipit-source-id: b0feaa02cee8a1ee94d032426d19c501ff3b2534