[INTERNAL] removed SessionIDObservable dependency

Removed SessionIDObservable dependency from the ProjectNegotiation
classes. Used session.getID() instead which also will never change
during runtime.

Change-Id: I006b501c3c7b7f74bfd7576c3d65cf2f91c3223d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1138
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>