Currently the listeners for disconnected and failed connection events
are unsubscribed as soon as the connection is established, so
the CONNECTION_DISCONNECTED is never triggered which would clear the
'connection' field. This commit will clear the 'connection' state on
CONNECTION_WILL_CONNECT. It's needed anyway given that there's no
guarantee on when and if the async disconnect operation will finish.

One issue caused by the 'connection' not cleared was that
CONNECTION_FAILED was not reduced correctly and the reload screen was
not displayed for the following scenario:
1. Join and leave any working conference.
2. Turn off network connectivity on the device.
3. Wait for CONNECTION_FAILED. The reload screen will not be displayed,
   because CONNECTION_FAILED is not reduced correctly, because the old
   'connection' value is still there.