Initialize /dev/urandom earlier in boot.

It's a security best practice to carry entropy across reboots.
(see "man 4 random"). Currently, entropy saving and mixing occur
in the system_server, via the EntropyMixer code. Unfortunately, the
EntropyMixer code runs fairly late in the boot process, which means
early boot doesn't have high quality entropy. This has caused security
problems in the past.

Load entropy data as soon as we can in the early boot process, so that
we can get /dev/random / /dev/urandom into a "random" state earlier.

Bug: 9983133
Change-Id: Id4a6f39e9060f30fe7497bd8f8085a9bec851e80