Add clatd to apex/com.android.tethering-file_contexts

clatd binary is starting to be shipped by apex since T+ release
and the shipped clatd is belong to u:object_r:clatd_exec:s0.

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

Change-Id: I4f6f0944e94e165983a19a5d3c3a117274f6bbac
diff --git a/apex/com.android.tethering-file_contexts b/apex/com.android.tethering-file_contexts
index 9398505..1b578ea 100644
--- a/apex/com.android.tethering-file_contexts
+++ b/apex/com.android.tethering-file_contexts
@@ -1 +1,2 @@
-(/.*)?           u:object_r:system_file:s0
+(/.*)?                  u:object_r:system_file:s0
+/bin/for-system/clatd   u:object_r:clatd_exec:s0