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/public/update_engine_common.te b/public/update_engine_common.te
index d110238..dd90fbc 100644
--- a/public/update_engine_common.te
+++ b/public/update_engine_common.te
@@ -65,6 +65,10 @@
 # apply / verify updates on devices mapped via device mapper
 allow update_engine_common dm_device:blk_file rw_file_perms;
 
+# read /dev/dm-user, so that we can inotify wait for control devices to be
+# asynchronously created by ueventd.
+allow update_engine dm_user_device:dir r_dir_perms;
+
 # read / write metadata on super device to resize partitions
 allow update_engine_common super_block_device_type:blk_file rw_file_perms;