Add initial preferred APN feature

In multi-sim scenarios, each sim will have its own preferred APN
setting. When ApnSettings sets/reads the preferred Apn, SUB id will
be passed to TelephonyProvider.  TelephonyProvider will update/return
the value of the key 'apn-id0' or 'apn-id1' based on the given SUB id.

Change-Id: Ifbbb49f2b38a5c6cff56785313a80a34479ea08f