aconfig: update c/c++ codegen

Two major changes to c/c++ codegen

(1) explicit setter for each flag instead of a generic flag setter
void override_flag(std::string name, bool val) is replaced with

void <flag name>(bool val) for each flag name

This has several advantages:
  (a) generated code is more c++ idomatic
  (b) no longer need to create flag name string constants
  (c) any typo in the code is caught early in the build time

(2) remove flag setter and flag override reset methods/functions when
generating code targets for production. If developers want to update
their main function to take command line arg for flag overrides, they
can use compile time macros to decide if the flag override code should
be included.

Bug: b/279483801
Test: atest aconfig.test
Change-Id: I6141f7f979b32fe0426154d578edeb997ae5ff7c