[INTERNAL] fixed thread access in WhiteBoardManager

Calling ColorUtils.setColor should be done in UI thread context.

Change-Id: I7c913acadb4491154ea332d71a0bd96dadfefd13
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1935
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>