system_server: bring back sdcard_type neverallow rule

We had disabled the neverallow rule when system_server was
in permissive_or_unconfined(), but forgot to reenable it.
Now that system_server is in enforcing/confined, bring it
back.

Change-Id: I6f74793d4889e3da783361c4d488b25f804ac8ba

system_server.te

@@ -385,4 +385,4 @@ allow system_server keystore:keystore_key {
 
 # Do not allow accessing SDcard files as unsafe ejection could
 # cause the kernel to kill the system_server.
-# neverallow system_server sdcard_type:file rw_file_perms;
+neverallow system_server sdcard_type:file rw_file_perms;