[INTERNAL] Trust IActivity.getSource() specification

This method cannot return null. Therefore any checks for null on its
return value were removed. In addition, the test utility class
"NetworkRequest" and related methods could be simplified by removing
unnecessary parameters and deleting dead code.

Change-Id: I05e94d64ee5b156de2d2d632d96da22ced30fa48
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1518
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>