Expose AID_KEYSTORE

Keystore uses this as the user ID for keys that it creates for its
own use.

Bug: 176450483
Test: keystore2_test
Change-Id: Ia698912917cd209eebcfc007a8badce7a39159d6
diff --git a/libcutils/Android.bp b/libcutils/Android.bp
index 0f3763c..a99cae2 100644
--- a/libcutils/Android.bp
+++ b/libcutils/Android.bp
@@ -363,8 +363,9 @@
     local_include_dirs: ["include"],
     bindgen_flags: [
         "--whitelist-function", "multiuser_get_app_id",
-        "--whitelist-function", "multiuser_get_user_id",
         "--whitelist-function", "multiuser_get_uid",
+        "--whitelist-function", "multiuser_get_user_id",
+        "--whitelist-var", "AID_KEYSTORE",
         "--whitelist-var", "AID_USER_OFFSET",
     ],
 }