[INTERNAL] AbstactAct.Prov.: fireActivity method is now protected final

Made this method protected and final so it cannot be used by other
components to generate activities by misusing subclasses of that class.

Change-Id: Ibc5933aa1661e5e781cd6226abed06050a564c3b
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1621
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>