Ceph is very robust for keeping data, beside using cephfs or exporting rbd. It is not too easy to access object directly with a client, let alone mobile. By using Rados gateway, android client can easily using S3/Swift http/s request to read object data from the internet.
It is a simple idea to use an existing open source android FileManager e.g. https://github.com/arpitkh96/AmazeFileManager