Reduce EntropyMixer seed size from 4096 to 512 bytes.

The suggested seed size from Linux drivers/char/random.c is 512 bytes
rather than the 4096 bytes currently used.

Change-Id: I3dd55105f0a95ce9cda34b4cf08987b8a28c4ac9
Merge "Reduce EntropyMixer seed size from 4096 to 512 bytes."
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
Merge "Make EntropyMixer mix in output of Hardware RNG into Linux RNG."