Hi Alex,
You can create your own GET and POST request calls directly to a FeatureService and extract the response data. Here is a link to our REST API documentation: ArcGIS REST API.
In regards to displaying that information top of a Google Android SDK map it depends on the type of feature data and what you want to do with it. You'll need to custom write all the code to handle the request/response translation with the feature service and converting the Esri features to Google shapes. Depending on what you are planning on doing this could end up being a ton of custom code that could be easily gotten via our SDK. You'll also lose a ton of built-in functionality by not using the ArcGIS Runtime SDKs.
As far as feature layer licensing you will have to check with the organization that is providing the feature layer for any conditions/restrictions/costs that they may have.
In terms of using the ArcGIS Runtime SDKs there are licensing requirements, here's a link: Legal Information | Licensing Terms. If you want to drill down into more specific SDK licensing questions I'd be happy to get you in contact with someone?
