Add getPreferredApn() in buildWaitingApns()

Now, only createAllApnList() calls getPreferredApn().
Unfortunately, in this time, getSubId() may not be proper value.
So getPreferredApn() could not get value which we want/set as preferred apn.
As a result, preferred apn is null and wrong apn info can be sent to modem
and be used by SETUP_DATA_CALL procedure.
To resolve this, just add getPreferredApn() in buildWaitingApns().

Bug: 22159434
Change-Id: Iee92325aa91c5c470328055746c321065a9829d4