Set the permission to file clatd and directory for-system

Set clatd privs to clat:clat:-r-sr-sr-x and set its parent
directory privs to root:system:dr-xr-x---. Makes sure that
only root and system group members can execute the binary.

Test: flash and check the file permission

$ adb shell ls -lZ /apex/com.android.tethering/bin
dr-xr-x--- 2 root system u:object_r:system_file:s0  4096 1970-01-01 08:00 for-system

$ adb shell ls -lZ /apex/com.android.tethering/bin/for-system
-r-sr-sr-x 1 clat clat u:object_r:clatd_exec:s0  24352 1970-01-01 08:00 clatd

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

Change-Id: I9537d47b135e6e0324fb40ece2b9f7befb159244

Tethering/apex/Android.bp

@@ -61,6 +61,7 @@ apex {
     binaries: [
         "clatd",
     ],
+    canned_fs_config: "canned_fs_config",
     bpfs: [
         "offload.o",
         "test.o",

Tethering/apex/canned_fs_config

+/bin/for-system 0 1000 0550
+/bin/for-system/clatd 1029 1029 06555