Fix MMS failure due to wrong apn merge.

Even after merging two apn entries with same apn,
DcTracker is using the unmerged preferred apn for
data call. This causes MMS apn request to start a
second data call request on the same apn.netd cannot
add routes if same network interface is used for
two separate network requests and results in MMS
transaction failure.
Fix it by merging the apns before getting preferred
apn setting.

Change-Id: I088dc3f7287c14306873bbbf906902a5d2a88533
CRs-Fixed: 819471