[CrOS Cellular] Fix issue where shill's Custom APN List field is cleared

Prior to this CL, if the user sets custom APNs, logs out, logs back in,
and then opens settings, the custom APN list field in shill is cleared.

This CL corrects this issue so that CrosNetworkConfig does not
unintentionally clear out the custom APN list from shill when settings
is opened.

Bug: b/162365553
Test: chromeos_unittest
Change-Id: Iee13491d745fe2264e482ed36eec399be1fbdae6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5626923
Reviewed-by: Gordon Seto <gordonseto@google.com>
Commit-Queue: Jason Zhang <jiajunz@google.com>
Reviewed-by: Jason Zhang <jiajunz@google.com>
Cr-Commit-Position: refs/heads/main@{#1319537}