Minor c/c++ codegen update

1, Moved "#include <string>" from exported header to test flag provider
header file.

2, For production target and read only flags, the generated c
api should just return default value instead of calling into c++ api

3, Remove using namespace server_configurable_flags from header, instead
of just having the namespace to be spelled out in each api call. Having
using namespace xxx in header is not a c++ best practice.

4, Replace #ifdef #def #endif with #pragma once

Bug: b/279483801
Test: atest aconfig.test
Change-Id: I3e55a7b14301f3de419795467f33e2dc889d371e