Add SEPolicy for PRNG seeder daemon.

Manual testing protocol:
* Verify prng_seeder daemon is running and has the
  correct label (via ps -Z)
* Verify prng_seeder socket present and has correct
  label (via ls -Z)
* Verify no SELinux denials
* strace a libcrypto process and verify it reads seeding
  data from prng_seeder (e.g. strace bssl rand -hex 1024)
* strace seeder daemon to observe incoming connections
  (e.g. strace -f -p `pgrep prng_seeder`)
* Kill daemon, observe that init restarts it
* strace again and observe clients now seed from new instance

Bug: 243933553
Test: Manual - see above
Change-Id: I0a7e339115a2cf6b819730dcf5f8b189a339c57d
diff --git a/private/file.te b/private/file.te
index 3f5531f..60e2274 100644
--- a/private/file.te
+++ b/private/file.te
@@ -120,3 +120,8 @@
 # This executable does not have its own domain because it is executed in the caller's domain. For
 # example, it is executed in the `artd` domain when artd calls it.
 type art_exec_exec, system_file_type, exec_type, file_type;
+
+# Filesystem entry for for PRNG seeder socket.  Processes require
+# write permission on this to connect, and needs to be mlstrustedobject
+# in to satisfy MLS constraints for trusted domains.
+type prng_seeder_socket, file_type, coredomain_socket, mlstrustedobject;