Create /metadata/password_slots during boot.

This directory is used to store the Weaver/GateKeeper slot map so GSIs
do not overwrite host keys in secure storage.

Bug: 123716647
Test: /metadata/password_slots exists after boot
Change-Id: Ib0ca13edec38e68cba1fc2124465571feedc4be7
diff --git a/rootdir/init.rc b/rootdir/init.rc
index ce4b380..7a826cc 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -397,6 +397,7 @@
     restorecon_recursive /metadata
     mkdir /metadata/vold
     chmod 0700 /metadata/vold
+    mkdir /metadata/password_slots 0771 root system
 
 on late-fs
     # Ensure that tracefs has the correct permissions.