sepolicy: Allow recovery to mount on tmpfs

/storage is a tmpfs volume, and is where updater stores its zip
when downloading updates.  Devices with emmc partitions that are
used as 'sdcard' volumes will end up with paths like:

  /storage/UUID/...../update.zip

where UUID is the mount point for the partition and update.zip is
the downloaded update.  With this change, minivold can create the
UUID folder and mount onto it, fixing the application of updates.

Change-Id: I4fa84fd590f5ff0f91e38c49cef0c179728fdf43