i found a source saying it is Hausa (a language and people living in the north of Nigeria) and a source saying it is Swahili, but both say the meaning is 'gift'.
here are the links:
http://www.babynamesworld.com/profile.php?seostats=1&name=Adia(this website says Adia is Swahili)
http://www.tzschoeckel.de/fnamesa.htm(this website says Adia is Hausa, note: 'ein Geschenk' is 'a gift' in
German, thus same meaning)
http://www.uiowa.edu/~africart/toc/people/Hausa.html(this website is about the Hausa people if you are interested)