shill: cellular: add CellularPco for extracting elements from PCOs This CL adds a CellularPco class to parse and extract the elements of a PCO structure from raw data. BUG=b:112664666 TEST=Run unit tests. Change-Id: I9e72b6ea4599de645cc0e8fba8372e5acac59a9e Reviewed-on: https://chromium-review.googlesource.com/1176659 Commit-Ready: Ben Chan <benchan@chromium.org> Tested-by: Ben Chan <benchan@chromium.org> Reviewed-by: Brian Norris <briannorris@chromium.org>