Fixed that emergency APN is a duplicate in APN list

Added check to prevent duplicate APN setting added into
the all APN list.

Test: Unit tests
Bug: 122741227

Merged-In: I28197e49868c2649416b5e9cc81b2144d41342d9
Change-Id: I28197e49868c2649416b5e9cc81b2144d41342d9
(cherry picked from commit 4297e6ee)