The aar file doesn't contain the transitive dependencies and doesn't have a pom file which describes the dependencies used by the library.
It means that, if you are importing a aar file using a
flatDir repo you have to specify the dependencies also in your project.
You should use a maven repository (you have to publish the library in a private or public maven repo), you will not have the same issue.
In this case, gradle downloads the dependencies using the pom file which will contains the dependencies list.
This works with aar libraries that are published to a remote or local maven repository, In your case it sounds like the library will not be published to even a local maven repository. I can't find any definitive information as to if it will work in your circumstances, but you should give it a shot.