Define GID range for external data and OBBs.

To quickly answer the question of "how much data is a UID using?" we
need a GID range to label files on external storage, similar to
the GID ranges already defined for cached and shared data.

Also define a new GID that will be used to label the OBB files shared
between all users under /data/media/obb.

Test: builds, newly added tests pass
Bug: 34263266
Change-Id: I16fb7d166c3dcdeafbf477162d191d3b73ae8ac2
diff --git a/include/cutils/multiuser.h b/include/cutils/multiuser.h
index 4f23776..5bd9c7b 100644
--- a/include/cutils/multiuser.h
+++ b/include/cutils/multiuser.h
@@ -32,6 +32,7 @@
 extern uid_t multiuser_get_uid(userid_t user_id, appid_t app_id);
 
 extern gid_t multiuser_get_cache_gid(userid_t user_id, appid_t app_id);
+extern gid_t multiuser_get_ext_gid(userid_t user_id, appid_t app_id);
 extern gid_t multiuser_get_shared_gid(userid_t user_id, appid_t app_id);
 
 /* TODO: switch callers over to multiuser_get_shared_gid() */