Restrict system_server_startup domain

This seems like an oversight when system_server_startup was
introduced (commit caf42d615dc9488d8e3601d3277167d61475a61a).

Test: Presubmits
Change-Id: Ia371caa8dfc2c250d6ca6f571cf002e25703e793
diff --git a/private/seapp_contexts b/private/seapp_contexts
index c7daf6b..d47134b 100644
--- a/private/seapp_contexts
+++ b/private/seapp_contexts
@@ -99,9 +99,11 @@
 # inputs are matched on a key value rule line.
 #
 
-# only the system server can be in system_server domain
+# only the system server can be assigned the system_server domains
 neverallow isSystemServer=false domain=system_server
+neverallow isSystemServer=false domain=system_server_startup
 neverallow isSystemServer="" domain=system_server
+neverallow isSystemServer="" domain=system_server_startup
 
 # system domains should never be assigned outside of system uid
 neverallow user=((?!system).)* domain=system_app