[CLATJ#3] libclat: move generateIpv6Address() from netd to apex

generateIpv6Address() 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: I42e152e6fe3ad577a0274e1d55e737318f61176d
2 files changed