Simplified the initial attach APN logic

1. Instead of using the hardcoded order, read the order
   from configuration. This also allows carriers to specify
   the allowed types of APN can be used for initial attach.
2. Removed the preferred APN logic. The preferred APN is
   specifically for the default APN after a successful data
   setup. It's not related to initial attach.

Fix: 169657628
Test: Manual
Change-Id: I9fbf4f270726cba1281770db88fec7c1bb62e6db