Codegen Android: build react-native-codegen via yarn when building from source

Summary:
The Gradle codegen integration requires the JS CLI to be first built via `yarn run build`. This commit puts that logic in a `build.sh` script, then defines a Gradle task to build it.

Changelog: [Internal]

Reviewed By: hramos

Differential Revision: D24556992

fbshipit-source-id: 7092de7c1126edc157b122f4b2243e55f7188846