Implement subscriberIdQuery API method

Open-WebOS-DCO-1.0-Signed-Off-By: Simon Busch <morphis@gravedo.de>
Cleanup implementation of the platformQuery API method from unneeded things

Open-WebOS-DCO-1.0-Signed-Off-By: Simon Busch <morphis@gravedo.de>
Move some code into correct source file instead of keeping in the main service one

Open-WebOS-DCO-1.0-Signed-Off-By: Simon Busch <morphis@gravedo.de>
ofono: implement subscriberIdQuery API method

Open-WebOS-DCO-1.0-Signed-Off-By: Simon Busch <morphis@gravedo.de>