shill: cellular: handle Verizon-specific PCO values

This CL extends CellularCapabilityUniversal to identify if a
Verizon-specific PCO value exists in a PCO update and translates the PCO
value into corresponding subscription state.

BUG=b:112664666
TEST=Run unit tests.

Change-Id: I7e93b707bc2d63bf556d5c089ab7fac0a3c63410
Reviewed-on: https://chromium-review.googlesource.com/1189431
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>