[REFACTOR] moved static utility methods out of RosterUtils class

Useless code *perfume* as always and again -> no GUI access in the
logic.

Change-Id: I998b01239206900f0eaa58bae38d61a4858fd08e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1315
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>