Stop manually mixing /dev/hw_random.

The kernel has been doing this itself since 2014. We still want the rest
of this code, though, both for devices without a hw_random device but
also to transfer entropy from one boot to the next (as described on the
man page, the link to which I've updated to avoid a redirect).

Also reorder includes to satisfy the upload hook.

Bug: http://b/179086242
Test: treehugger
Change-Id: I68ac8e04191b71339708b18eee646a2d65063fd5