aconfig: create c++ interlop from rust flag apis

Using cxx to generate c++ code for the apis that are marked to exported
in the ffi mod in src/lib.rs. For generated code simplicity, it returns
a new flag struct for each rust api.

Create a cc_library to wrap around generated c++ code. See the created
aconfig_storage.hpp for the exported header. Note there is a nested
aconfig_storage::test_only_api namespace, where we place test only apis.
The production apis are placed under aconfig_storage namespace.

Bug: 321077378
Test: m libaconfig_storage_cc
Change-Id: I73a85a26d3749908abc461362f5e0bfc3f85bf4d