aconfig: read from new storage in Rust codegen

Read storage values for read-only flags, and log failures or mismatches
with compiled-in defaults. Continue to return values fetched from the legacy storage.

Test: cargo t
Bug: 328444881
Ignore-AOSP-First: internal storage migration, this code will be deleted
Change-Id: I4651617270129794dec64e324b4aa7836cce7a43