Updates publish-to-s3 plugin to 0.7.0 and publishes '-sources.jar'

[oguzkocer]

This PR updates publish-to-s3 Gradle plugin to 0.7.0 and uses the newly introduced androidSourcesJar task to upload the -sources.jar file so it can be used by developers to access the source code from their IDEs. More details can be found in: https://github.com/Automattic/publish-to-s3-gradle-plugin/pull/29

To test:

• https://github.com/wordpress-mobile/WordPress-Android/pull/15492 can be used to test the same changes applied to WordPress-Utils-Android & WordPress-FluxC-Android projects.

[peril-automattic[bot]]

You can test the changes on this Pull Request by downloading the APK here.

[oguzkocer]

I am going to open a PR to test these changes and then merge the PR in.