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Make EntropyMixer mix in output of Hardware RNG into Linux RNG.

On devices with Hardware RNG (HW RNG), this makes EntropyMixer
periodically (every three hours) read 512 bytes from HW RNG and mix
them into the Linux RNG.

This is done without increasing Linux RNG's entropy estimates to
avoid having to trust the quality of the HW RNG. This approach cannot
reduce the quality of the Linux RNG output, but may increase it.

Change-Id: Ifde851004301ffd41b2189151a64a0c5989c630f