Allow update_verifier to read dm blocks

Update_verifier will read dm-wrapped system/vendor partition. Therefore,
change the sepolicy accordingly.

Here's the denied message:
update_verifier: type=1400 audit(0.0:131): avc: denied { read } for
name="dm-0" dev="tmpfs" ino=15493 scontext=u:r:update_verifier:s0
tcontext=u:object_r:dm_device:s0 tclass=blk_file permissive=0

Bug: 34391662
Test: Read of /dev/block/dm-0 succeeds during boot time.
Change-Id: I23325bd92f6e28e9b1d62a0f2348837cece983d1

public/update_verifier.te

@@ -13,8 +13,8 @@ allow update_verifier block_device:dir search;
 allow update_verifier ota_package_file:dir r_dir_perms;
 allow update_verifier ota_package_file:file r_file_perms;
 
-# Read all blocks in system partition.
-allow update_verifier system_block_device:blk_file r_file_perms;
+# Read all blocks in dm wrapped system partition.
+allow update_verifier dm_device:blk_file r_file_perms;
 
 # Use binderized HAL
 hwbinder_use(update_verifier)