Use explicit flatbuffer IDs in y2017 and newer.

Non-explicit ids are risky.  We've seen backwards incompatible
changes...

Change-Id: Id6ceebe031ac80430191f367635d0e951c3d2cbc
diff --git a/y2019/status_light.fbs b/y2019/status_light.fbs
index 665b3f8..cbc65b3 100644
--- a/y2019/status_light.fbs
+++ b/y2019/status_light.fbs
@@ -2,9 +2,9 @@
 
 table StatusLight {
   // How bright to make each one. 0 is off, 1 is full on.
-  red:float;
-  green:float;
-  blue:float;
+  red:float (id: 0);
+  green:float (id: 1);
+  blue:float (id: 2);
 }
 
 root_type StatusLight;