Wait for ueventd cold boot

Without this we may start prng_seeder too early.

With this patch the sequencing seems deterministically correct:
init: Wait for property 'ro.cold_boot_done=true' took 37ms
init: processing action (early-init) from (/system/etc/init/prng_seeder.rc:5)
init: starting service 'prng_seeder'...

Bug: 306665006
Test: Start VM, inspect logs
Change-Id: Ib009b879a9a68107c21a1915ff84e86f2b931acf
diff --git a/microdroid/init.rc b/microdroid/init.rc
index f5f3f15..4cc0475 100644
--- a/microdroid/init.rc
+++ b/microdroid/init.rc
@@ -30,6 +30,11 @@
     # We don't directly exec the binary to specify stdio_to_kmsg.
     exec_start init_debug_policy
 
+    # Wait for ueventd to have finished cold boot.
+    # This is needed by prng-seeder (at least).
+    # (In Android this happens inside apexd-bootstrap.)
+    wait_for_prop ro.cold_boot_done true
+
 on init
     mkdir /mnt/apk 0755 root root
     mkdir /mnt/extra-apk 0755 root root