Add Android assets packaging project and process

[jmartinesp]

I believe this is everything we'd need to publish the Android assets as an AAR:

1. There is a new platforms/android folder which contains the Android project with the minimum of changes needed to publish the library as an AAR into the GH gradle registry.
2. This new folder also includes a script to build the assets, copy them to the right folder and trigger the publishing gradle task.
3. And there is a new publish_android job that will call this script and automate the publishing.

There is still the need to update the version in the gradle.properties file before a release so it matches the EC one. That can probably be done later either manually, through some script that changes the version for both, or with a script that extracts the value from package.json and overrides it here (although the value there seems to be 0.0.0?).

Note this PR still needs the relative paths for assets to work.