[connman] service: mark mobile data connection as "auto"

When connectionagent "helps" connman to switch to mobile data, such
connection was treated by connman as a "user" connection. That
interferes with the connman's auto-connection logic. Particularly,
the "user" connections are not automatically brought down - connman
seems to expect the "user" who established the connection to bring it
down when the connection is no longer needed. Which connectionagent
never does, so mobile context never gets deactivated. Occasionally,
some data is still transferred over mobile data connection even if
wifi is up and operational. That's not a kind of behavior the user
would expect, especially if he/she is being charged for the mobile
data traffic.

This counter-hack allows connman to apply its auto-disconnect logic
to the mobile data interface activated by connectionagent.