onc: Handle translation of Cellular.APNList property.

This CL adds code to onc_translator_shill_to_onc.cc that handles a list of
nested objects and uses it to translate the Cellular.APNList property.

BUG=280359

Review URL: https://codereview.chromium.org/24248005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228602 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228602