Update how aconfig cache is used for codegen

The storage backend now supports DEVICE_CONFIG. Thus needs to update
utility function like should_include_flag to also exclude flags that
have the storage backend set to DEVICE_CONFIG. This also implicitly
implies assign_flag_ids to exclude DEVICE_CONFIG backed flags.

Moved some common logic to src/codegen/mod.rs. Added ensure statements
in each codegen module to ensure that flag ids are assigned properly.

In native codegen module cpp.rs and rust.rs, ensure that no DEVICE_CONFIG
backed flag can be used in codegen. In java codegen module java.rs, for
each flag, add a field to indicate if using device config or not.

The actual codegen change will come after this change.

Bug: b/406508083
Test: atest -c
Change-Id: I3c6692b7453f87e2ee7f51a10faf8131e9020afc