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/keystore.te b/private/keystore.te
index 5cded8a..85f1517 100644
--- a/private/keystore.te
+++ b/private/keystore.te
@@ -23,3 +23,4 @@
 # Keystore need access to the keystore_key context files to load the keystore key backend.
 allow keystore keystore2_key_contexts_file:file r_file_perms;
 
+get_prop(keystore, keystore_listen_prop)