aconfig: add c codegen

c codegen can be done at the same time with cpp codegen, the idea is to
create a c compatible header that defines the flag apis, including flag
override apis for test. then in the corresponding cpp file, the
implementation simply calls into cpp api.

c header supports static method interface, and unit test override, but
it does not support injection pattern compared to cpp header

Bug: b/279483801
Test: atest aconfig.test
Change-Id: Ie62b76d6524e443de5d3c2f9000f7f66623ab571