Set relative_install_path to "for-system"

Used for allocating clatd under $(apex_path)\bin\for-system\clatd

Bug: 212345928
Test: manual test
1. Connect to ipv6-only wifi.
2. Try IPv4 traffic.
   $ ping 8.8.8.8

Change-Id: Ibfb83db738a67d266856966042486edc57473649
diff --git a/Android.bp b/Android.bp
index 3ef0115..70b525b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -67,6 +67,7 @@
     shared_libs: [
         "liblog",
     ],
+    relative_install_path: "for-system",
 
     // Static libc++ for smaller apex size while shipping clatd in the mainline module.
     // See b/213123047