aconfig: add a new aconfig storage file flag listing api

added a new function called list_flag_with_info to list all the flags
given all four storage files (package.map, flag.map, flag.val, flag.info).
also exported this api thru cxx interface so aconfigd can use it.

Bug: b/312444587
Test: atest -c
Change-Id: Ibbfe657b980d40e25e5e28962b930338192e2d98
diff --git a/tools/aconfig/aconfig_storage_file/build.rs b/tools/aconfig/aconfig_storage_file/build.rs
index 1feeb60..e0ade2a 100644
--- a/tools/aconfig/aconfig_storage_file/build.rs
+++ b/tools/aconfig/aconfig_storage_file/build.rs
@@ -14,4 +14,6 @@
         .inputs(proto_files)
         .cargo_out_dir("aconfig_storage_protos")
         .run_from_script();
+
+    let _ = cxx_build::bridge("src/lib.rs");
 }