init sets keystore.boot_level, keystore reads

Bug: 176450483
Test: init can set, and keystore2 read, keystore.boot_level
Test: `adb shell getprop -Z | grep boot_level` returns
      [keystore.boot_level]: [u:object_r:keystore_listen_prop:s0]
Change-Id: Iedb37db19e9153995800fc97de6ee8c536179caa
diff --git a/private/init.te b/private/init.te
index 02d45a1..ca32e6f 100644
--- a/private/init.te
+++ b/private/init.te
@@ -79,3 +79,6 @@
 
 # Only init can write ro.property_service.version
 neverallow { -init } property_service_version_prop:property_service set;
+
+# Only init can set keystore.boot_level
+neverallow { -init } keystore_listen_prop:property_service set;