Restrict access to /dev/hw_random to system_server and init.

/dev/hw_random is accessed only by init and by EntropyMixer (which
runs inside system_server). Other domains are denied access because
apps/services should be obtaining randomness from the Linux RNG.

Change-Id: Ifde851004301ffd41b2189151a64a0c5989c630f
diff --git a/file_contexts b/file_contexts
index c4c4a09..dfdb439 100644
--- a/file_contexts
+++ b/file_contexts
@@ -48,6 +48,7 @@
 /dev/full		u:object_r:full_device:s0
 /dev/fuse		u:object_r:fuse_device:s0
 /dev/graphics(/.*)?	u:object_r:graphics_device:s0
+/dev/hw_random		u:object_r:hw_random_device:s0
 /dev/input(/.*)		u:object_r:input_device:s0
 /dev/iio:device[0-9]+   u:object_r:iio_device:s0
 /dev/ion		u:object_r:ion_device:s0