Pull Element and FieldElement out into functions

This makes them easier to reuse in the future and a little less coupled.

Change-Id: Ib70319ed91381e0b30f4e87fbfe6d262c1c0f179
diff --git a/aos/flatbuffer_utils.cc b/aos/flatbuffer_utils.cc
new file mode 100644
index 0000000..1060fa8
--- /dev/null
+++ b/aos/flatbuffer_utils.cc
@@ -0,0 +1 @@
+#include "aos/flatbuffer_utils.h"