Settings: Add the read only APN feature.

Disable the edit screen for read only APNs

Some operators don't want their APNs to be edited by
end user to avoid mistaken changes. End user should
be able to only view the APNs, but not edit.

Change-Id: I89f26ccc687d6989c562f164dbe53961a4d8a057
CRs-Fixed: 774852