Fix preferred apn with specific RAT user added is erased

After a new default APN with a proper RAT is added and chosen as peferred,
both cellular and IWLAN DcTrackers retrive the APN changed event, generally
just there is one DcTracker satisfied to handle preferred APN check logic,
but another DcTracker also checks if it is statisfied with the transport's
network, hence, it always fails, so that the preferred APN is deleted wrong.

To fix it, let the proper transport's DcTracker handle if the preferred APN
needs to be erased.

Change-Id: I623ec485099c31919a84a666a100dbcfeac803ca
CRs-Fixed: 2564107