Restore labels on /postinstall during recovery.

This patch mirrors what was done in the main init.rc to relabel
/postinstall.

Bug: 27178350
Bug: 27177071

(cherry picked from commit 6bcc8af6e5a5bf9cc0987305cdfa24d4f6e4afa9)

Change-Id: I8320559f014cfb14216dcc350e016fc1db05cb14
diff --git a/etc/init.rc b/etc/init.rc
index dc18659..5915b8d 100644
--- a/etc/init.rc
+++ b/etc/init.rc
@@ -1,6 +1,9 @@
 import /init.recovery.${ro.hardware}.rc
 
 on early-init
+    # Set the security context of /postinstall if present.
+    restorecon /postinstall
+
     start ueventd
     start healthd