init: do expand_props before calling the builtins.

Also switch expand_props to std::string.

Bug: 22654233

Change-Id: I62910d4f74e2b1a5bd2b14aea440767a2a8462b7
diff --git a/init/init.h b/init/init.h
index c166969..d812733 100644
--- a/init/init.h
+++ b/init/init.h
@@ -138,7 +138,7 @@
 extern struct selabel_handle *sehandle;
 extern struct selabel_handle *sehandle_prop;
 
-void build_triggers_string(char *name_str, int length, struct action *cur_action);
+std::string build_triggers_string(struct action *cur_action);
 
 void handle_control_message(const char *msg, const char *arg);