Set /system/xbin permissions to 750.

This directory contains only the su binary which is executable only by
root and shell uids, so need not be accessible to other users.

Test: Device boots
Test: adb shell ls -ld /system/xbin
Change-Id: I4c9daab68b29832ef0ace2dec274687e4496da81
diff --git a/libcutils/fs_config.cpp b/libcutils/fs_config.cpp
index b9fc82e..a0fc039 100644
--- a/libcutils/fs_config.cpp
+++ b/libcutils/fs_config.cpp
@@ -85,7 +85,7 @@
     { 00751, AID_ROOT,         AID_SHELL,        0, "system/bin" },
     { 00755, AID_ROOT,         AID_ROOT,         0, "system/etc/ppp" },
     { 00755, AID_ROOT,         AID_SHELL,        0, "system/vendor" },
-    { 00751, AID_ROOT,         AID_SHELL,        0, "system/xbin" },
+    { 00750, AID_ROOT,         AID_SHELL,        0, "system/xbin" },
     { 00751, AID_ROOT,         AID_SHELL,        0, "system/apex/*/bin" },
     { 00751, AID_ROOT,         AID_SHELL,        0, "system_ext/bin" },
     { 00751, AID_ROOT,         AID_SHELL,        0, "system_ext/apex/*/bin" },