shill: cellular: clean up subscription state logic

The MMModem3gppSubscriptionState enum is being deprecated from
ModemManager. This CL migrates the private
CellularCapabilityUniversal::SubscriptionState enum into a top-level
SubscriptionState enum, and uses it in place of
MMModem3gppSubscriptionState.

BUG=b:112664666
TEST=Run unit tests.

Change-Id: I5430eff2fd013cd452fdd17e9fd1a8fc21c3b36f
Reviewed-on: https://chromium-review.googlesource.com/1189428
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>