Add a C++ library wrapper for the android.security flags

Adds a C++ library for the android.security flags, so the feature flags
can be accessed from native code.

This CL is paired with flagging of storage area setup code in
installd and vold, which use the `unlocked_storage_api` feature flag.

Bug: 325129836
Test: atest StorageAreaTest
Change-Id: Ib702b9f0725ac0d960d393e3ff9b2d6d18fc934c

AconfigFlags.bp

@@ -258,6 +258,11 @@ java_aconfig_library {
     defaults: ["framework-minus-apex-aconfig-java-defaults"],
 }
 
+cc_aconfig_library {
+    name: "android_security_flags_aconfig_c_lib",
+    aconfig_declarations: "android.security.flags-aconfig",
+}
+
 // UsageStats
 aconfig_declarations {
     name: "android.app.usage.flags-aconfig",