on some phones the onBackendConnected finishes prior to the onActivityResult()
leading to the pending photo uri being cleared before processing the result.

this leads to 'Take photo' not working.

but we probably don’t need to clear the photo uri if there is to activiyResult
to clear as well

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