Determine unprivileged helper in exec-tool and fix invocation

lomiri-push-service currently determines the path of unprivileged helpers provided by click apps itself and then abuses URI arguments in order to pass it into lomiri-app-launch. Since this is actually the job of exec-tool, refactor both so that the unprivileged helper is looked up inside exec-tool and passed to lomiri-app-launch via lomiri_app_launch_helper_set_exec(). This results in the unprivileged helper actually being invoked with the correct arguments (fixes #39 (closed)). This has no fallback for reading hook file directly but depends on the click hook regenrating the JSON file containing all helpers for all click apps with push notification support.