We were expiring subscribers during active calls. This is because
the T3212 is stopped under certain conditions but we didn't stop
that timer at all.
Remember if T3212 timer was stopped due something done by NITB and
update the expiration time at the end of the radio connection, as
the phone should restart it when returning to MM Idle.
It is a bit difficult to decide when we should set the flag. E.g.
in a CM Service Request we don't know if we accept the service and
during a LU we already send MM messages before we accept or reject
the subscriber.
The easiest is to set the flag when receiving a paging response
on known subscribers and at the end of the authentication process.
Do not expire a subscriber that has an active connection that is
marked with the flag, e.g. we would still expire a subscriber that
is being paged.
Manual tests executed/passed:
* gst LUTest.st verified that a expiration date was set
* gst SMSTest.st (doing another LU but forcing a timeout on the
SMS sending). Verified that the expire_lu was updated.
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