[FIX] XMPPTransmitter disposes connection too early

The order of creation of objects inside the PICO Container
may be different depending on their dependencies.

Ensure that the connection is only "disposed" on NOT_CONNECTED
and/or ERROR states so that it is possible to send final
messages when the connection is going to be disconnected.

Change-Id: Ia04975c4f24bbc80f00cab343e4254dcd2e69d7d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/848
Tested-by: Jenkins CI
Reviewed-by: Patrick Schlott <pschlottk@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>