Add SELinux policy for using userfaultfd

ART runtime will be using userfaultfd for a new heap compaction
algorithm. After enabling userfaultfd in android kernels (with SELinux
support), the feature needs policy that allows { create ioctl read }
operations on userfaultfd file descriptors.

Bug: 160737021
Test: Manually tested by exercising userfaultfd ops in ART
Change-Id: I9ccb7fa9c25f91915639302715f6197d42ef988e
diff --git a/private/system_server.te b/private/system_server.te
index 8bee1bf..9ef8363 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -12,6 +12,8 @@
 # Define a type for tmpfs-backed ashmem regions.
 tmpfs_domain(system_server)
 
+userfaultfd_use(system_server)
+
 # Create a socket for connections from crash_dump.
 type_transition system_server system_data_file:sock_file system_ndebug_socket "ndebugsocket";