[FIX] invoking stop on ActivitySequencer may stall forever

Moral of the lesson -> always use

synchronized(foo)
{
   while(!condition)
      foo.wait()
}

Change-Id: Ifa74d0de72916729d622dabea08cbea6d9f06c95
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/927
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>