clatd: change to pass in tun filedescriptor via command line

Test: atest clatd_test, built and installed on aosp_blueline device
  connected to ipv6-only wifi network: ping 8.8.8.8 still works
  and it is via v4-wlan0 clat tun interface

Bug: 65674744
Bug: 131268436
Signed-off-by: Maciej ┼╗enczykowski <maze@google.com>
Change-Id: I8c9e235e9a5bf1a1436e8dc3af8d0aa86f6dc1a5
Merged-In: I8c9e235e9a5bf1a1436e8dc3af8d0aa86f6dc1a5
(cherry picked from commit 716518d9b9ca52182498b1d7ed0f0ed8ab42cbe1)
7 files changed