netd: Active FTP Helper

Enable conntrack helper for active FTP connections when
tethering is started.

Bug:69265432
CRs-fixed:2136660

Test: as follows
      - Boot device - Pass
      - Start tethering - Pass
      - Start active FTP on tethered host (USB) - Pass
      - Download file over FTP - Pass
      - make -j64 netd_unit_test
            adb shell /data/nativetest64/netd_unit_test/netd_unit_test - Pass

Change-Id: I5c812afe9602e80ea1c42d0cafba40fe6019a995