Create sysfs_hwrandom type.

HwRngTest needs access to the hwrandom sysfs files, but untrused_app
does not have access to sysfs.  Give these files their own label and
allow the needed read access.

(cherry-pick from internal commit: 85c0f8af)

Bug: 27263241
Change-Id: If572ad0931a534d76e148b688b76687460e99af9