Simplify formatting of cell data

This is more of a tentative patch, as it breaks the dual-SIM case. Thanks @illarionov for pointing it out!.

Do we want to support dual-SIM devices (I think we do)? Even if we do, we should think about how to deal with that case: do we want to do separate submissions for each type of cell and then one for wifi, or maybe bundle wifi along with one of the cell submissions, and does it even matter? And what should the "radio" attribute be in that case? cc @hannosch