Clear IPv4 address on tethering teardown.

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
    - wlan0 in STA mode, wlan1 up/down in AP mode
      no lingering IPv4 mode
    - USB tethering up/down works
    - bluetooth tethering yields:
05-18 17:50:49.726   719   756 D TetherController: untetherInterface(bt-pan)
05-18 17:50:49.729  1194  1230 E Tethering: [bt-pan] ERROR Failed to clear IPv4 address on interface bt-pan: java.lang.IllegalStateException: command '224 interface setcfg bt-pan 0.0.0.0 0' failed with '400 224 Failed to clear address (No such device)'
      which is acceptable (no actual crash, just a log message)
Bug: 79905644
Change-Id: Iaf29788a6692d810f3160e3f21d06b7452ecbaa6