Setup a Gradle build inside `ReactAndroid/hermes-engine` to download Hermes sources

Summary:
This Diff sets up a small Gradle build inside `ReactAndroid/hermes-engine`
The idea is to kickoff a small project where we can download Hermes sources and start a compilation of
the Hermes sources from there.

Specifically the used paths are:

- `/sdk/hermes` for the unzipping
- `/sdk/download/hermes.tar.gz` for the tarball location
- `/sdk/hermes/.hermesversion` for the hermes version.

allow-large-files

Changelog:
[Internal] [Changed] - Setup a Gradle build inside `hermes-engine` to download Hermes sources

Reviewed By: hramos

Differential Revision: D34210236

fbshipit-source-id: 97034f5608dfb3fcd1d74e9851944f7a60e52ea1