Only process SELinux contexts if SELinux is not disabled
    
    When operating under SELinux disabled, do not try to retrieve SELinux
    context of the D-Bus sender because it will fail with a D-Bus error
    message that confuses users in the logs and breaks operations.
    
    Resolves: rhbz#1578150
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>