[CLATJ#5] libclat: move detect_mtu() from netd to apex

detect_mtu() is moved from ClatdController without behavior
change. The unique_fd for socket fd is replaced by primitive int
because libbase is not supported in mainline.

Bug: 212345928
Test: build and boot
      atest libclat_test
Change-Id: Ib9c1a9d4b9e1c141d88164e8489c5044fdf70685
4 files changed