if the activity is not connected during finish it won’t receive the last end user state.

this code remembers it even if the actual session is already gone. so when activity reconnects and
we can’t find the real rtp session we can look up the last state instead.

the state machine in jingle file transfer does not prevent that the connection
is being finished twice

* Fix typo

* Update README.md

This should be good enough to survive some network switches where both networks are online at the same time to allow for some handover
(for example when enabling wifi the 3G connection will usually (probably depends on OS) live on for a moment