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
1 file changed