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/TEST_MAPPING b/tools/aconfig/TEST_MAPPING
index b7ff8ef..421e94a 100644
--- a/tools/aconfig/TEST_MAPPING
+++ b/tools/aconfig/TEST_MAPPING
@@ -97,5 +97,9 @@
     }
   ],
   "postsubmit": [
+    {
+      // aconfig_storage file cpp integration tests
+      "name": "aconfig_storage_file.test.cpp"
+    }
   ]
 }