Now that the keep-alive instance defaults to ALIVE by default, we can
always arm it when starting to activate the active-connection.

The keep-alive instance may have been armed earlier already:
for example, when binding its lifetime to a D-Bus name or
when watching the connection's visible state.

However, at the moment when we queue the active-connection for
activation, we also want to make sure that the keep-alive instance is
armed. It is nicer for consistancy reasons.

Note, that nm_keep_alive_arm() has no effect if nm_keep_alive_disarm()
was called earlier already. Also note, that NMActiveConnection will
disarm the keep-alive instance, when changing to a state greater than
ACTIVATED. So, all works together nicely.

Also, no longer arm the keep-alive instance in the constructor of
NMActiveConnection. It would essentially mean, that the instances
is aremd very early.

Also, as alternative point of interest, arm the keep-alive instance
when registering the signal handler in "nm-policy.c".