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/property_contexts b/private/property_contexts
index 5933e30..ea7488d 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -1071,6 +1071,9 @@
 # TODO remove this property when Keystore 2.0 migration is complete b/171563717
 persist.android.security.keystore2.enable    u:object_r:keystore2_enable_prop:s0 exact bool
 
+# Broadcast boot stages, which keystore listens to
+keystore.boot_level u:object_r:keystore_listen_prop:s0 exact int
+
 partition.system.verified     u:object_r:verity_status_prop:s0 exact string
 partition.system_ext.verified u:object_r:verity_status_prop:s0 exact string
 partition.product.verified    u:object_r:verity_status_prop:s0 exact string