[UI] SingleUserChat is no longer colored with session user colors

This chat is session independent. It does not add any value to change
the color if the remote user in not session but may create unwanted
noise.

The chat now uses always the same colors regardless of the state of the
local and/or remote participant.

Change-Id: I62656605e059af15f536ff88ae5df88e6ca5c9b9
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1727
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>