Allow update_engine to inotify_add_watch dm-user device nodes.

inotify_add_watch requires read permissions and these were only granted
to the /dev/block/dm-user directory, not the device nodes.

Denial: avc:  denied  { read } for  pid=1918 comm="update_engine" name="product_b-user-cow" dev="tmpfs" ino=162 scontext=u:r:update_engine:s0 tcontext=u:object_r:dm_user_device:s0 tclass=chr_file permissive=0

Bug: 238572067
Test: apply OTA
Change-Id: I3fa7c9600873f4a2638fd140287511005f5aac1d
diff --git a/public/update_engine_common.te b/public/update_engine_common.te
index e8fd29e..12961e7 100644
--- a/public/update_engine_common.te
+++ b/public/update_engine_common.te
@@ -72,6 +72,7 @@
 # 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;
+allow update_engine dm_user_device:chr_file r_file_perms;
 
 # read / write metadata on super device to resize partitions
 allow update_engine_common super_block_device_type:blk_file rw_file_perms;