Sepolicy for migrating storaged to statsd

This adds /proc/uid_io/stats to the files that system server is able to
read.

Test: Manual test on master produces no selinux violations.
Change-Id: I2c7afec149f893b000094739d91531dec559de6f

private/system_server.te

@@ -775,6 +775,7 @@ allow system_server {
   proc_pipe_conf
   proc_stat
   proc_uid_cputime_showstat
+  proc_uid_io_stats
   proc_uid_time_in_state
   proc_uid_concurrent_active_time
   proc_uid_concurrent_policy_time