Add sepolicy for starting the snapuserd daemon through init.

Restrict access to controlling snapuserd via ctl properties. Allow
update_engine to control snapuserd, and connect/write to its socket.

update_engine needs this access so it can create the appropriate dm-user
device (which sends queries to snapuserd), which is then used to build
the update snapshot.

This also fixes a bug where /dev/dm-user was not properly labelled. As a
result, snapuserd and update_engine have been granted r_dir_perms to
dm_user_device.

Bug: 168554689
Test: full ota with VABC enabled
Change-Id: I1f65ba9f16a83fe3e8ed41a594421939a256aec0
diff --git a/private/property.te b/private/property.te
index ffd3c51..575785d 100644
--- a/private/property.te
+++ b/private/property.te
@@ -1,5 +1,6 @@
 # Properties used only in /system
 system_internal_prop(adbd_prop)
+system_internal_prop(ctl_snapuserd_prop)
 system_internal_prop(device_config_profcollect_native_boot_prop)
 system_internal_prop(device_config_storage_native_boot_prop)
 system_internal_prop(device_config_sys_traced_prop)