Read from new storage in C++ aconfig codegen

Read from the new aconfig storage backing in C++ aconfig codegen, for
READ_ONLY flags. Log if there is a match with the legacy storage, or a
mismatch, or a failure.

Only enabled if instrumentation is enabled, which will be enabled by a
build flag in a follow-up CL.

Test: m && cargo t
Bug: 328444881
Change-Id: I691dfad8860d8f917e93c5d56dac19f8791de943