init.rc: Lock down access to /proc/net/fib_trie

Make /proc/net/fib_trie only readable to root.

Bug: 31269937
Test: Device boots, file has appropriate permissions.
Change-Id: I0d01ce5c043d576344a6732b0b9ff93d62fcaa34

rootdir/init.rc

@@ -148,6 +148,9 @@ on init
     write /proc/sys/net/ipv4/conf/all/accept_redirects 0
     write /proc/sys/net/ipv6/conf/all/accept_redirects 0
 
+    # /proc/net/fib_trie leaks interface IP addresses
+    chmod 0400 /proc/net/fib_trie
+
     # Create cgroup mount points for process groups
     mkdir /dev/cpuctl
     mount cgroup none /dev/cpuctl cpu