aconfig: update c/c++ codegen

1, Current codegen would still include an empty flag value cache even if
there are no read write flags. This is important as the flag value cache
is a std::vector. Thus adding this cache would automatically pull in
libc++ dependency which we should not.

2, There lacks of c/c++ codegen unit tests to lock down the generated
code when a flag package contains only read only flags. It should not
include server_configurable_flags header. And it should not include the
flag cache.

Bug: b/316614694
Test: atest aconfig.test
Change-Id: Iee4366672932ee507694f5fc73b526c66fcf0e9b