net: conn_mgr: Simplify blame handling

Don't track up/down blame separately.
Instead, track a single last-blame iface.

Don't track blame iface inside set_ready.
Instead, track directly inside handle_update.

These two changes will simplify the addition
of blame for IPv4- and IPv6-specific events.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>