init.rc: Reland "mkdir /data/misc/vold"

Creation of /data/misc/vold was first introduced by commit 25775e8e. It
then got reverted, probably inadvertently, by:

 commit 2e24bcfd
 Author: Paul Lawrence <paullawrence@google.com>
 Date:   Fri May 22 18:31:55 2015 +0000

     Revert "Set up crypto for user directories in init."

This folder is required by vold. In its absence, if one tries to format
an SD card as internal storage, an error will occur, with the following
message printed to logcat:

 01-08 06:31:48.389  1002  1069 E vold    : Failed to persist key

Restoring /data/misc/vold fixes this bug.

This fix has been verified on the emulator, which currently does not
allow its virtual SD card to be adopted, but another CL will be uploaded
to enable that.

Change-Id: Ibb87ee821630a038a6f9b7b6a9da50fe03ca690d
Signed-off-by: Yu Ning <yu.ning@intel.com>