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

rootdir/init.rc

@@ -177,6 +177,9 @@ on post-fs-data
     # We restorecon /data in case the userdata partition has been reset.
     restorecon /data
 
+    # Avoid predictable entropy pool. Carry over entropy from previous boot.
+    copy /data/system/entropy.dat /dev/urandom
+
     # Create dump dir and collect dumps.
     # Do this before we mount cache so eventually we can use cache for
     # storing dumps on platforms which do not have a dedicated dump partition.