DcTracker: verify preferred apn list is not empty before enabling

If a non-null but empty cursor is returned mCanSetPreferApn will be
set to true even though the preferred apn list is empty. Verify the
cursor contains at least one apn before setting mCanSetPreferApn.

Change-Id: I6a66ab334d997d8ced06ea0b596ba3463f4ce85b