flattened apex: fix /apex/com.android.tethering/bin/for-system{,/clatd} mode

from packages/modules/Connectivity/Tethering/apex/canned_fs_config
   /bin/for-system 0 1000 0750
   /bin/for-system/clatd 1029 1029 06755
the Tethering apex mounts at /apex/com.android.tethering,
but that's a bind mount, it really lives at /system/...
in the flattened fs image.

Testing via:

aosp$ export OVERRIDE_TARGET_FLATTEN_APEX=true
aosp$ a_make_target aosp_cf_x86_phone-userdebug

$ sudo mkdir /mnt/point && sudo mount -o loop,ro /aosp/out/target/product/vsoc_x86/system.img /mnt/point && egrep /mnt/point /proc/mounts && sudo ls -dlnZ /mnt/point/system/apex/com.android.tethering/bin/for-system{,/clatd} && sudo umount /mnt/point && sudo rmdir /mnt/point

/dev/loop0 /mnt/point erofs ro,relatime,user_xattr,acl,cache_strategy=readaround 0 0

drwxr-x---. 2 0 1000 u:object_r:system_file:s0 44 Apr 11 03:12 /mnt/point/system/apex/com.android.tethering/bin/for-system
-rwsr-sr-x. 1 1029 1029 u:object_r:clatd_exec:s0 18876 Apr 11 03:12 /mnt/point/system/apex/com.android.tethering/bin/for-system/clatd

while without this patch it showed:

drwxr-x--x. 2 0 2000 u:object_r:system_file:s0 44 Apr 11 01:53 /mnt/point/system/apex/com.android.tethering/bin/for-system
-rwxr-xr-x. 1 0 2000 u:object_r:clatd_exec:s0 18876 Apr 11 01:53 /mnt/point/system/apex/com.android.tethering/bin/for-system/clatd

Test: TreeHugger
Bug: 273821347
Bug: 277646103
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I9714b1e935afbe27c2eb4be3f96a9742eb752e92