Apart from the intellectual value in taking control of an aircraft , if indeed it was possible in theory, from a smart phone, if any hijackers were interested in actually accomplishing an end result of physical control, just go to the link below for a far more reliable method.
I see several pages of posts on the theoretical aspects of ACARS control etc but not much with respect to this below,
See if this is a more practical and possible eventuality.
WJA
http://youtube/mlmzvF2qkDY