Merge apns after prefer apn is set

Apns with similar fields are being merged to a single apn
in DcTracker which will affect prefer apn setting if the
preferred apn is merged to some other matching apn.

To avoid this merge apns will have to be done after prefer apn is
set.

Change-Id: I72d1b28c8530b46c6ba47c7134dd722ae163fb03
CRs-Fixed: 778225