Let the kernel hwrng thread manage hw_random mixing.

This has been something the kernel does automatically since 2014, so
there's no obvious reason to add extra work during boot to duplicate
that effort.

Bug: http://b/179086242
Test: treehugger
Change-Id: I44cce99a892e4f2a6a303c2126bd29f955f5fb23
3 files changed