Improve starter_cmd status output

This makes "starter_cmd status" output the time since an application
was started. If you specify an application, it outputs
the application name, time since last started, and state.

Change-Id: Ibccaa19f12c8e7c4cdde384f9594ab7c00ae5224
diff --git a/aos/starter/starter_rpc_lib.cc b/aos/starter/starter_rpc_lib.cc
index 89fc2c2..20f6f5a 100644
--- a/aos/starter/starter_rpc_lib.cc
+++ b/aos/starter/starter_rpc_lib.cc
@@ -127,18 +127,15 @@
                       aos::starter::ApplicationStatus>::Empty();
 }
 
-const aos::FlatbufferVector<aos::starter::Status> GetStarterStatus(
+std::optional<const aos::FlatbufferVector<aos::starter::Status>> GetStarterStatus(
     const aos::Configuration *config) {
   ShmEventLoop event_loop(config);
   event_loop.SkipAosLog();
 
   auto status_fetcher = event_loop.MakeFetcher<aos::starter::Status>("/aos");
   status_fetcher.Fetch();
-  if (status_fetcher) {
-    return status_fetcher.CopyFlatBuffer();
-  } else {
-    return FlatbufferVector<aos::starter::Status>::Empty();
-  }
+  return (status_fetcher ? std::make_optional(status_fetcher.CopyFlatBuffer()) :
+      std::nullopt);
 }
 
 }  // namespace starter