Reverted to the old logic for APN selection

Reverted to the old logic that we select APN based on
RAT, but adding a special handling for 2G cases. This can
deal with the case that some APN settings has EHRPD which
we incorrectly thought that APN setting can be used on CDMA
network.

Bug: 134086103
Test: Unit tests
Change-Id: I557f00d32f53498782394ea7824e316e263ca7e7