Support mainline beta production mode codegen

Mainline beta production mode codegen will use device config as storage
backend. Also, the starting implementation will not cache flag values in
the codegen. This is more in line with legacy flags. Mainline developers
have the best flexibity this way. In some use cases, the flag values are
cached in the code. In some cases (like flag read each time wifi turns
on), flag value are not cached and always read from storage.

Bug: b/406508083
Test: m and added unit test point
Change-Id: I201cb886cf972a71b61c7bb0428c675028db13b0