Permit writing to /dev/random and /dev/urandom.

Apps attempting to write to /dev/random or /dev/urandom currently
succeed, but a policy violation is logged. These two Linux RNG
devices are meant to be written to by arbitrary apps. Thus, there's
no reason to deny this capability.

Bug: 10679705

Change-Id: Ife401f1dd2182889471eef7e90fcc92e96f9c4d6