SMS in chrome browser is handled over D-Bus. go/dbus-in-mustash

See SMSClient / GsmSMSClient / ModemMessagingClient

Some devices with built-in cellular require receiving an SMS message to activate the service.
Wrapper class NetworkSmsHandler. Used by ash in TraySMS.

Per benchan: We only show received SMS via notifications. The current implementation directly hooks into the DBus interfaces exposed by cromo / ModemManager, and not via shill. So the functionality is pretty much detached from the rest of the networking related bindings.

Plan: Factor out SMS handling from the rest of network handling, then use the client library in ash alone.