Allow wificond to drop signals on hostapd

Stopping hostapd abruptly with SIGKILL can sometimes leave the driver
in a poor state.  Long term, we should pro-actively go in and clean up
the driver.  In the short term, it helps tremendously to send SIGTERM
and give hostapd time to clean itself up.

Bug: 30311493
Test: With patches in this series, wificond can cleanly start and stop
      hostapd in integration tests.

Change-Id: Ic770c2fb1a1b636fced4620fe6e24d1c8dcdfeb8