SELinux policy: let vold write to device:dir.

I have no idea what vold is doing when this operation is attempted
(when a full-disk encrypted device is booting up). Thus, I don't know
if there is a better way of restricting the policy.

Change-Id: I537b70b1abb73c36e5abf0357b766292f625e1af

vold.te

@@ -7,6 +7,7 @@ typeattribute vold mlstrustedsubject;
 allow vold system_file:file x_file_perms;
 allow vold block_device:dir create_dir_perms;
 allow vold block_device:blk_file create_file_perms;
+allow vold device:dir write;
 allow vold devpts:chr_file rw_file_perms;
 allow vold rootfs:dir mounton;
 allow vold sdcard_type:dir mounton;